This is an interesting app as it is packed with with different instruments and many parameters. It is meant to be used in conjunction with the developers Hardware Oval (hangdrum) instrument.
It is for free and if you want a library of sounds with an easy to play interface...

Google Play:
(So do not see it on the Play Store... DO not know why. Have it installed since some time... So feel weird posting this but here I am posting it anyway if somebody finds it on the play Store or if somebody have made an APK of this free app to share).

Will install it now...

Google Play:
play.google.com/store/apps/details?id=com.rfo.harpejji&referrer=utm_source%3DBlog%26utm_medium%3DApp%26utm_campaign%3Dsale

A harpejji is a neat stringed instrument that has frets like a guitar but is finger tapped. The strings are tuned in whole steps so it is possible to quickly transpose a melody to a new key by maintaining its "shape". C notes are designated with a dot.

This app isn't meant to substitute the far superior real harpejjis but to give users a chance to learn and experiment with their unique chromatic layout.

Choose landscape (better) or portrait mode and number of frets (I prefer 6-8).

You can learn more about real harpejjis at https://www.marcodi.com

Some tips on how to play:
https://www.marcodi.com/pages/learn-to-play

Google Play:
play.google.com/store/apps/details?id=com.rfo.harpejji&referrer=utm_source%3DBlog%26utm_medium%3DApp%26utm_campaign%3Dsale

​A harpejji is a neat stringed instrument that has frets like a guitar but is finger tapped. The strings are tuned in whole steps so it is possible to quickly transpose a melody to a new key by maintaining its "shape". C notes are designated with a dot.

This app isn't meant to substitute the far superior real harpejjis but to give users a chance to learn and experiment with their unique chromatic layout.

Choose landscape (better) or portrait mode and number of frets (I prefer 6-8).

You can learn more about real harpejjis at https://www.marcodi.com

Some tips on how to play:
https://www.marcodi.com/pages/learn-to-play

Installed this the other day and it is a nice instrument that allows for bends and slides with a player friendly layout. Would recommend for anyone that are using virtual instruments for one more flavor.

Google Play:
play.google.com/store/apps/details?id=com.csquad.muselead

MuseLead is a multi-touch synthesizer performance surface to play music. It's a musical instrument that simulates the electric guitar. It has an interface similar to the IOS instrument GeoShred. 
With MuseLead, you can achieve guitar sounds such as continuous bends, sweepings, shredding, tappings and more..
You can edit presets with a complete list of guitar pedals to achieve original tones.

Important notes :
MuseLead is a musical instrument that needs low latency for better performance. However, Android devices are not known for the best user experience in terms of audio latency, therefore, we invite you to check your device latency in the link below before downloading.
https://superpowered.com/latency

Muselead should work perfectly with the following devices :
- Google Pixel
- HTC Nexus 9
- Nexus 6P
- Nexus 5X

We also tested the application on these devices :
- Samsung Galaxy S9
- Samsung Galaxy S8
- Huawei Nova 3i
- Huawei P20 Pro
- Hp Slate 8 Pro
- Samsung Galaxy J series

The application runs also on these devices but with lower performance
- Samsung Galaxy Tab A SM-T555
- Samsung Galaxy S7 
- Samsung Galaxy S5 Neo
- Asus Laser 3

For all other devices please compare your audio latency in the link above.

Nice app for playing instruments with different expressions for each instrument available using tilt. And very nice being for free and without ads! 
It will be nice to see where this will go in the future as it is great as is.

Google Play:
play.google.com/store/apps/details?id=com.castlewrath.hexpress

Hexpress is a collection of musical instruments for your phone. You can use it for learning, playing and composing music whenever you have the time - on train, while waiting in line and during boring meetings. Using headphones is strongly recommended for louder and better sound quality, and to not disturb others around you. The application has simple, colorful and clean interface designed to be accessible to young children.

While each instrument behaves differently in some way, in general notes are played by touching shapes on the screen, and sound is shaped by tilting the phone left-right and up-down. Different instruments have different effect controls - fade in, reverb, wah, tremolo...

Most of Hexpress instruments have unusual honeycomb note arrangement that's sometimes called "harmonic table note layout". It has many interesting properties compared to standard piano layout:

• effective utilization of device screen (3+ octaves range)
• note relations (intervals) are uniform across whole range; to transpose song to different key just play same patterns on different part of instrument
• most chord shapes are grouped tightly and they can be executed with single finger swipe
• in typical scale and melody runs, notes are alternated between fingers of two hands, so they can be played with speed and precision
• large intervals are as accessible as smaller intervals

Hexpress is without ads, free and open-source. It's also work in progress - interface, sounds and features are all subject to change. The app doesn't collect any information about user and cannot access internet. Storage access permission allows any user to add and modify sounds and other instrument behaviour. Feedback is much appreciated.

This app is made to work together with a midi controller that is based on the instrument called hang instrument. It seems to be a very nice midi controler but it costs a lot of money...
Luckily for us there is this app that is well made and have loads of virtual instruments inside that is laid out like a hang instrument. So it is of course tied into scales as there is not the full twelve notes. This is an advantage as it makes improvisation easy and the octave changes are easy to implement. Some effects are built in and recording as well.

The sounds are good and there is no ads and free!
Also like that there is midi out, so can be used to control hardware.
One thing to point out is that having this layout breaks the normal piano thinking and at least for me it makes me come up with a different type of playing rhythmically and melodically.
So for live recording very interesting. (Depending on device/latency)

Recommended download.

Google Play:
https://play.google.com/store/apps/details?id=com.ovalsound.Oval

The Oval App is a music, synthesis and effects platform designed from the ground-up
to revolutionize music creation, learning and live performance.

When paired together with the Oval physical controller, the App allows you to take full
advantage of each subtle gesture, combining the expressiveness
of an acoustic instrument with the flexibility of modern digital synthesis.

A NEW ESSENTIAL FOR MUSIC CREATORS
Tweak your favorite sounds from the growing collection of high quality instruments by adding effects such as filters, delays, and reverb. All the instruments in the library have been carefully crafted by professional sound designers, making the most of the Apps powerful gesture mapping functionality.

HOT FOR MUSIC ENTHUSIASTS
The Oval is also designed for those new to music creation. The intuitive layout will make it easy for you to immerse yourself in the music, and the large built-in scale collection allows you to explore different moods while always staying in key.

SHARE YOUR CREATIONS
When inspiration strikes, record your creations and share them with your friends, or export them to another music app and take them to the next level.

TECHNICAL FEATURES
- Exclusive sound libraries
- X/Y sensitive performing pads
- Large (and growing) list of scales from around the world, including microtonal scales
- Customizable filter, delay, reverb and dynamics effects
- Recording feature with simple editing capabilities like trim and fade in/out
- Compatible with other apps that use MIDI
- Customizable A4 frequency

Interesting virtual instrument and help-out for guitar player is now free so why not give a twirl?

Google Play:
https://play.google.com/store/apps/details?id=com.pedrocorp.android.guitar

Robotic Guitarist is both a guitar emulator and music toolbox for guitar players and musicians in general and a fun and enjoyable musical playground for everyone. Even if you don't know a word about music or can't play the guitar! Just play and it will sound good!



★ Amazing guitar emulator

Choose a chord without having to learn finger positions and just hit the strings. You'll enjoy realistic visual effects and high fidelity sound. You can strum, play arpeggios or any other desired technique. You can play licks as well.

★ The guitar lab (never-ending possibilities)

Have you ever wondered how a reverb-enabled, fuzz-distorted bell would sound?

♪ Lots of different sounds
Pick among a large categorized set of sounds including a number of kinds of guitars and playing modes and the sounds of all sorts of musical instruments. Make the guitar sound as a flute or a piano! Funny and exotic sounds are included too, like those from animals. Or even pick a different one for each string! What about creating a band with a human chorus for the lows and a violin for the highs?

♪ Easy-to-use effects
Create a nice atmosphere and add deepness with impressive sound effects like reverb, flanger and echo. Each one features a list of variants so it's plug and play!

♪ Versatile distortion
Would you like a heavier or harsher sound? No problem! Pick a kind of distortion from the several available and set an intensity level. Your electric guitar will be absolutely electrifying!

★ Chord guide

An easy-to-use interface allows you to change your chord selection on the go. You'll see at all times a diagram showing finger positions. Very useful for learners or for those who play already and need to recall a chord.

★ Custom chords and chord presets

You'll be able to take music as far as your imagination takes it because you have the possibility of creating your own chords with a easy-to-use editor. They don't have to be actual guitar chords, but any layout of notes whose sound you like. You'll be able to play those songs with weird hand positions, since you are not limited to standard chords. You can define free strings you can play licks on. This feature allows, for example, to follow a tablature with much more fidelity.

Furthermore, you'll be able to save the chord set you have been composing so you can recover it later, with no need of defining it again chord by chord. That way you'll have at hand every chords needed for each song or experiment.

★ Reliable guitar tuner

If you play a non-virtual guitar, ‮‪‮‪‮‪‮‪Robotic Guitarist is your loyal electronic pocket tuner. It's able to detect frequencies with high accuracy.

★ Handy digital metronome

It has settings for tempo (BPM) and different time signatures. Keep the rythm!

I am happy to see this developer doing some more apps as I was thinking the other day that maybe he stopped doing Android apps. He have made some other interesting apps with a lot of character. (He is involved in one bigger project as well that I will write about in the post after this.) Some of the apps like this one is for sitting and recording as they do not come with built in recording but I think that is totally okay and will lead to more organic interactions with the music.

Googe Play Info:

Rainstick

Rainstick is an audio/visual physics simulation of a rainstick. Several simulations are available with different kind of physical parameters, geometries etc.

It has been built with :
- libgdx an open source game framework : https://libgdx.badlogicgames.com/
- gdx-pd a wrapper of libpd for libgdx (libpd being an open source audio engine based on Pure-data) : https://github.com/mgsx-dev/gdx-pd

Thanks to mgsx for his help and the awesome libgdx tools he built for the making of this application ! http://www.mgsx.net/

This is a couple of reviews of this interesting virtual instrument.
One in English and two in Spanish.

https://www.techwibe.com/robotic-guitarist-android-app/

http://androidayuda.com/2016/05/13/tocar-la-guitarra-android-robotic-guitarist-lo-conseguiras/
​
​http://www.todoandroid.es/index.php/173-aplicacion/2326-robotic-guitarist-la-app-definitiva-para-amantes-de-la-guitarra.html​

A virtual instrument that has been around for a while now.
There is some interesting things that can be done with this instrument as placing different instruments on each string and play a whole little mini band at the same time live.
Yes it is not only guitar sounds and for this it can be a good option for guitarists looking for virtual instruments.

Google Play info:

​Robotic Guitarist is both a guitar emulator and music toolbox for guitar players and musicians in general and a fun and enjoyable musical playground for everyone. Even if you don't know a word about music or can't play the guitar! Just play and it will sound good!

★ Amazing guitar emulator
Choose a chord without having to learn finger positions and just hit the strings. You'll enjoy realistic visual effects and high fidelity sound. You can strum, play arpeggios or any other desired technique. You can play licks as well.
★ The guitar lab (never-ending possibilities)
Have you ever wondered how a reverb-enabled, fuzz-distorted bell would sound?
♪ Lots of different sounds
Pick among a large categorized set of sounds including a number of kinds of guitars and playing modes and the sounds of all sorts of musical instruments. Make the guitar sound as a flute or a piano! Funny and exotic sounds are included too, like those from animals. Or even pick a different one for each string! What about creating a band with a human chorus for the lows and a violin for the highs?
♪ Easy-to-use effects
Create a nice atmosphere and add deepness with impressive sound effects like reverb, flanger and echo. Each one features a list of variants so it's plug and play!
♪ Versatile distortion
Would you like a heavier or harsher sound? No problem! Pick a kind of distortion from the several available and set an intensity level. Your electric guitar will be absolutely electrifying!
★ Chord guide
An easy-to-use interface allows you to change your chord selection on the go. You'll see at all times a diagram showing finger positions. Very useful for learners or for those who play already and need to recall a chord.
★ Custom chords and chord presets
You'll be able to take music as far as your imagination takes it because you have the possibility ofcreating your own chords with a easy-to-use editor. They don't have to be actual guitar chords, but any layout of notes whose sound you like. You'll be able to play those songs with weird hand positions, since you are not limited to standard chords. You can define free strings you can play licks on. This feature allows, for example, to follow a tablature with much more fidelity.
Furthermore, you'll be able to save the chord set you have been composing so you can recover it later, with no need of defining it again chord by chord. That way you'll have at hand every chords needed for each song or experiment.
★ Reliable guitar tuner
If you play a non-virtual guitar, Robotic Guitarist is your loyal electronic pocket tuner. It's able to detect frequencies with high accuracy.
★ Handy digital metronome
It has settings for tempo (BPM) and different time signatures. Keep the rythm!

I like that it has midi import and have a deeper set of functions compared to many other virtual instruments.
Have not tried it tough...
​but it will be in the back of my head when I need some non western flavor!

Google Play info:

Gender Wayang is a 10-key metallophone, coming in pairs, most be doubled from an other pair one octave higher, tuned in Slendro scale. As Gender Wayang is played with two panggul (mallets) - other than the Gangsa in Gong Kebyar - two player together play the melody (left hand) and the interlocking parts kotekan (right hand) as a complete „mini“ Gamelan.Gamelan Gender is intended as a practicing tool. Play the polos part, while Gamelan Gender plays the sangsih, loop parts to memorize, generate your own songs as midi file in a sequencing program, import it into Gamelan Gender. Its keys can be tuned, to match your real instruments - this is important, as every set of Gender Wayang has its own tuning.
Comes with 8 original songs (gending) and a Gender Wayang and a Gangsa Pelog tuning.
• MIDI file player
• Animated keys for „traditional" learning
• Sound bank sampled from real instruments
• Keys assignable to midi notes (adjust to your MIDI file)
• Tune it to your real instruments
• Mathematical tuning system, only 5 keys have to be defined
• Calculated tuning can be overwritten, and each of the 40 keys frequency can be defined
• Loop parts for practicing, increase/decrease tempo on every re-loop
• Set proportional tempo changes for different parts...
• Change tuning to pelog scale and use it for Gong Kebyar practicing as well, create your own 10 key tunings....

So Samsung have released Virtual Reality glasses together with Oculus Rift and that looks very interesting if you have one of their new devices. There already exists a lot of apps/games/movies and pornography to be able to play and view. Having looked at the reviews of the glasses and apps it seems that it is working pretty good and can only imagine how far this will have gone in just a couple of years more!
Anyway there also exists a app for music making called Soundscape.
Do not feel that the app in itself seems very exciting or good for music making as it is not as hands on to input notes plus that as an app it seems pretty basic, but it is a beginning and soon we can all stand knee high in modular gear in a spaceship jamming with people all over the universe.

​Website:
http://www.creativeunitycoder.com/

So the representative of IK Multimedia have said that these Keyboard apps do not contain any poop-up ads...
Here is the Play Store info for the iGrand Piano and the iELectric is very similar:

THE CONCERT QUALITY PIANO APP

iGrand Piano is the ultimate grand piano instrument that puts an extraordinary collection of ultra-realistic acoustic pianos at your fingertips. Use the iRig KEYS and KEYS PRO portable keyboards (www.irigkeys.com) and you’ll be ready to jump start your mobile playing with the absolute best sounding piano instruments for your mobile world. Edit your sounds and save up to 4 user variations per instrument.Jam out, get inspired and capture your creativity — you can record your playing with Samsung Soundcamp if you have a Samsung Professional Audio Device.

iGrand Piano puts the world's finest pianos in your pocket with studio-grade quality that stands up to the most powerful virtual instruments available on a computer.

With high-definition stereo samples across multiple velocities, iGrand Piano brings you the most playable, expressive and beautiful sounding piano instruments available on a mobile device that you can play anytime and everywhere.

iGrand Piano gives you 8 world-class pianos (including one by registering).

Add the Piano Expansion 1 via in-app purchase to extend your piano library with 9 additional high-quality instruments or the Piano Expansion 2 with 22 all-new instruments including the Hollywood Piano, piano + strings layers and harpsichords.

Record your rehearsals or performances using iGrand Piano as a plug-in for Soundcamp, you can change the tempo of your piece or even choose to use a different piano AFTER you've completed your performance!

iGrand Piano also includes professional MIDI performances to help you audition the various piano instruments or just inspire your own creative muse!

The easy-to-use instrument editor puts real-time controls on the panel just above the keys to let you adjust the amount of ambience, the position of the lid (open, closed or anything in between), brightness, envelope release, key transposition and tuning.

iGrand Piano is designed to work seamlessly with most Android compatible MIDI controllers like IK Multimedia’s ultra-portable 37 mini-key iRig KEYs keyboard and super-compact 37 full-sized key iRig KEYS PRO keyboard. An OTG to Micro USB cable is required for connection and can be purchased separately from the IK online store.

iGrand Piano features:

- World-class, multi-velocity stereo grand and upright piano instruments (Upright Piano available by registering) 
- Expandable libraries offer 31 additional instruments instantly available via in-app purchase
- Intuitive instrument edit controls including ambience, lid, brightness, release, tuning and transpose
- Easy-to-use instrument editor with transpose to adjust ambience, tonality and tuning with 4 user variations per instrument
- Available as a plug-in for Soundcamp
- Low-latency for real-time playing (requires Android 5 Lollipop)
- More than 64-voice polyphony on the latest devices
- Can be used with Android-compatible MIDI controllers like iRig KEYS and iRig KEYS PRO
- Free version also available

To use iGrand Piano with Soundcamp, an Android device that supports Samsung Professional Audio technology is required, like the Samsung Galaxy S6, S6 Edge, Note 4 and Galaxy Note Edge or the Samsung Galaxy S5 or Galaxy Note 3 with Android 5 Lollipop installed.

For more information:

www.ikmultimedia.com/igrandpianoandroid
www.irigkeys.com

So this very interesting midi controller based on a Hang Drum.
Create Digital Music have tested it and they really like it-
http://createdigitalmusic.com/2015/06/played-oval-digital-hang-drum-amazing/

Yes it will cost a fair amount of money (that seems to be worth it though) but there is good news for us poor souls and that is that there will be a Android application that is meant to go with the hardware but after speaking to one of the developers he sent me the info and it looks like a nice virtual instrument just on it's own.

The Oval App is still in development. But I can tell you what is the Oval App now and what is going to be in for sure.
The Oval App has several sound sets and scales, hang sounds, percusive sounds and some other sounds, at the moment we have a sitar and several fx/pad sounds.
The Oval App allows to apply effects like delay to the sounds played.
The Oval App will have a four track looper to loop the notes played with the Oval.
We are working to allow the user to upload their own sounds.
The Oval App for iOS will be the first to incorporate Bluettoth connectivity with the Oval. We are working to integrate this capability in Oval App for Android too.


The Oval App will be usable without the Oval, but that makes sense only to prepare settings to play the Oval as the Oval App will lack the sensitivity to pressure and velocity and neither the ample space to develop the body gesture way to play the Oval.

Ha ha this must be the first time there is a posting with a real crap app!
Have used this in some music as I like the cheesy but beautiful sound and want to start to mention some of the virtual instruments as well.
This is basically a app that have sampled some electronic choir and packed it in with a keyboard. Full of publicity to! 
That said it is a specific sound that for me is useful and will use it more than once.

Play Store link:
Best Piano Choir

Here is an interesting proposition if you happen to be in Berlin in the beginning of August.
Yes the workshop will be in English so do not worry if you do not know German...
It is meant for iOS / Android / Win 8 devices and apps so that is all good.
I think that the best I can do is to post the information directly from the University website and with a link afterwards if you find it interesting and want to sign up.
___________________________________________________________________________________________________
App Music: 
Mobile Music Making for Professionals 
Lecturer: Matthias Krebs

date: August 4th - August 7th
place: UdK Berlin, Bundesallee 1-12
fee: 350 €
No. of participants: 9 to 15
language: English
application deadline: July 17th

A 4-day workshop on music making with mobile apps including experimentation, composition, production and performance. A creative space for enthusiasts, sound artists, music technologists and professional musicians exploring inspiration and creativity. Findings and results culminate in a final concert.

--------------------------------------

For quite a while now, making music with apps on smartphones and tablets has been more than simply a novelty. Some excellent music apps like Audiobus, TC-11, SECTOR, Beatsurfing, Mitosynth, Beatmaker2, Nave and many more offer innovative digital musical instruments and powerful production tools to users. These apps already allow the creation of remarkable musical results, as demonstrated by serious album productions and an increasing number of stage performances in recent years.

The workshop is conceived as a composition and sound workshop in the form of an experimental laboratory; a creative space for exchange and new discoveries in the multifaceted cornucopia of available apps and hardware. Enthusiasts, experts and interested parties from all backgrounds are therefore invited to participate in the collaborative investigation of and experimentation with sound, music and performance – a collective ‘learning by doing’ approach.

Project groups will organically develop performances, productions and/or composition pieces over the course of the workshop. The (partial) results of the interactive creative activities will be presented in a public performance at the c-base.

Participants
The workshop is aimed at enthusiasts, professional musicians, sound artists and music technologists with advanced experience creating music with apps. This is a workshop for those who are seeking an interactive exchange with others with regard to new musical forms, new apps, new hardware technology and innovative, expressive methods of sound making, performance and collaboration. Interested parties of all disciplines are encouraged to participate; diverse and varied backgrounds often bring valuable eclectic elements into the collaborative mix.

Participants may certainly play their own devices (iOS/android/Win8) during the workshop, however a large and varied supply of additional devices, peripheral equipment and amplifiers will also be provided. Participants are thus encouraged to mix, match, integrate, collaborate and experiment with a multitude of devices and apps during the course.

Matthias Krebs is a professional app musician, physicist, opera singer, multi instrumentalist and scientist. For many years now, he has also been active in theater production as a sound artist and composer. He founded the DigiEnsemble Berlin in 2010. Today it is the first professional music ensemble that regularly performs with mobile apps on physical stages (http://digiensemble.de).
__________________________________________________________________________________________________
Link to sign up:
Mobile music making workshop

Here is the Youtube channel for the Workshop / Seminar where you can see a lot of what happened last year:
https://www.youtube.com/channel/UCJD0dtU82XFFvdf1O61xtXw

The application Walk Band that is for free is good for anyone looking to have a bunch of virtual instruments in the same application (plus a solid set of functions to go with them) and here is a whole band performing their instruments using Walk Band. It is tight even if the music is not nothing to be so excited about... 
Actually it is a great watch and there is even a guitar solo!

Play Store link:
Walk Band

The developer is asking for people to help him out and try this Beta version downloadable directly from the Play Store for free maybe it is a good idea so we can influence the developer to make something good!
One thing that would be very useful for an application like this would be to have separate midi export. It is an application well suited for composing in a fast way and if it had separate midi export it would be easy to import into any DAW or Caustic (Caustic being one or the only application that can import seperate midi files into separate tracks). 
As I see it one thing that makes it hard to finish actual songs is that it is easy to get distracted with all the funky things you can do with effects, automation and just sculpting your sounds in the sound generators / Synthesizers. If there was a General Midi composer where the distraction did not exist and you could easily export to separate midi files there could be a big improvement in workflow.
So it is Beta and the developer needs help so please download it and give him suggestions!
(One thing that i noticed in this Beta version is that you have to tap twice on the menu items which can trip you up and dismiss the app.)

He made another app before this that is interesting to and it is a sequencer app with a few select instruments making it into good sketch pad. It is more directed to sketching out music for a pop / rock band than electronic music but can of course be useful for anything and it offers a few things that you do not see in other apps.
Here is a video of his other application (for free and no publicity).

Play Store links:
Music Riff Studio 
Music Riff Studio 2 Beta