I want to thank Andy for bringing this to my attention and it must be good news for anyone using Studio One as their DAW. Being an official app from PreSonus they use the same setup as the DAW and they made it for free, it seems like a perfect controller if using Studio One.

Google Play:
https://play.google.com/store/apps/details?id=com.presonus.dawremote

PreSonus® Studio One® Remote is a free Android remote control app designed specifically for use with PreSonus Digital Audio Workstation Studio One 3 Artist* and Professional on Mac® and Windows® computers. It’s the perfect companion, both as a “second screen” app in a workstation setup or as a flexible mobile remote for recording, mixing and editing when away from the computer.

Studio One Remote is based on the PreSonus software applications framework and utilizes the UCNET protocol for network connectivity and remote control. This is the same technology that powers PreSonus remote control apps such as UC Surface, as well as the popular multitrack live recording software Capture™ (desktop) and Capture for iPad.

Key Features:

- Remote control of Studio One 3 transport and mix console
- Commands page for access to all Studio One factory and user commands and macros
- Control up to 28 plug-in parameters using Control Link
- PreSonus UCNET networking technology for ultra-fast connectivity
- Macro Control view for quick access to FX parameters
- Fast song navigation using scalable timeline, marker list and Arranger sections
- Control any Studio One system on the same network; control a single Studio One with multiple remote apps simultaneously
- Start Page with demo mode and integrated Quick Help
- Access multiple Cue Mixes with independent faders
- Access record modes, Pre-count and Metronome settings
- Remote control over critical PreSonus hardware functions (currently supported products: Studio 192-series USB 3.0 interfaces, Quantum Thunderbolt™ interfaces, StudioLive AI-series mixers, and StudioLive Series III mixers)

REQUIREMENTS:

Compatible with Android devices running Lollipop 5.0 or later and a screen resolution of 1024 x 768 or higher. The Android device must be connected to the same wireless network as a Mac or Windows computer running Studio One 3 Professional.
* Studio One 3 Artist support requires the Studio One Remote add-on. Studio One add-ons are available at shop.presonus.com.

So this interesting controller is on sale and is now 11.99€/13.57$ until September 12.
Google Play:
https://play.google.com/store/apps/details?id=net.liine.lemurapp

Lemur is the world's best MIDI/OSC controller app, now available on Android!Benefiting from nearly 10 years of development, Lemur is synonymous with multitouch music control. All these major artists can't be wrong: Daft Punk. Björk, Plastikman, CHVRCHES, Glitch Mob, Sub Focus, Max Cooper, Barker & Baumecker, Orbital and many more.
Version 5.0 introduces the incredible new Canvas object and the Sequencer family of features with rock solid timing.
Canvas — Canvas is a brand new object that lets you script custom widgets with full multitouch support. The object is adapted from the HTML5 Canvas element, which means you can design widgets featuring complex shapes, animation, shadows, and transparency The Canvas object opens the door to a completely new Lemur experience, where the appearance and behaviour of objects can be designed to precisely match the demands of your project. Polygons, waveforms, circular objects and infinitely more are all possible with Canvas.
Sequencer — Lemur 5.0 introduces a family of new features for tight sequencing. This includes three new objects: StepNote, StepSwitch and StepSlider. Each of these provides a different flavour of sequencing and is ready to use right out of the box. The objects do the hard work for you, simple add them to your template and they are ready to control your MIDI instruments.
Skins — Lemur features three new skins, in addition to the classic look. Choose the look that fits your style and get inspired.
Connectivity — Lemur Android supports OSC and MIDI over Wi-Fi, as well as over USB with 3rd party tethering utilities.
REQUIREMENT (for the Lemur Daemon): 
- OS X (Intel only) 10.5+ 
- Windows (XP SP3 or 7 recommended)
Android is a vast platform and Liine had to choose which devices to support from the very beginning. Lemur is officially supported on Nexus 7 - this means that if you buy it for this device, Liine are committed to getting you up and running. It has also been tested on various Android 4.2+ devices with positive results. The list of officially supported devices will grow in the near future.
You can see a list of all devices users have installed Lemur on the Liine Wiki Docs:
https://liine.net/wiki/android_devices
Simply connect your Android device to your laptop and make the connection!

Interesting if you are a guitar player and have Android 6.01 to see more info about compatability go here:
https://jamstik.zendesk.com/hc/en-us/articles/217913348-Will-My-Android-Device-Work-With-Jamstik-

Google Play info:

​The jamstik+ is a digital SmartGuitar with real strings and real frets. It connects wirelessly to Android®, iPad®, iPhone® and Mac® and works with a multitude of music apps for beginners and pros alike. The jamstik+ senses where your fingers are before you even pluck a string which makes learning incredibly intuitive via our included guitar teaching curriculum. It’s always in tune, and is a breeze to connect wirelessly with your device via Bluetooth LE. The jamstik+ is more than a guitar. It’s also a MIDI controller, which means it can sound like almost any instrument you can think of. It’s also portable enough to put in your backpack, carry-on luggage, or briefcase so you can play anywhere.jamstik+ is the official companion app designed to establish your Android/iPad/iPhone connection to the jamstik device. The jamstik+ app also monitors your jamstik+ for battery status, firmware updates, and connection protocol selection.
The jamstik+ app has over 15 sounds to choose from and features an extensive guitar chord and guitar scale library which makes it easy to practice guitar with instant feedback. You can also fine-tune the performance of your jamstik+ with string sensitivity settings, one-touch custom tunings, and more. The portable nature of the jamstik+ and your mobile device means you can plug in headphones and practice in private.
For more information on the jamstik+, please visit our website at www.jamstik.com

So McDonalds have done something that I begrudgingly must admit is cool...
This is something that happened in Netherlands and it was only for one day.
They have used conductive ink that got printed on Place mats as can be seen above that can be used to control and record a music app that goes with Android and iOS.
If I was living in the Netherlands guess I would have had to eat one of their disgusting hamburgers to play with this! But have a feeling that this will happen in some other countries as well so maybe keep an eye out on the local McDonalds...
Unfortunately conductive ink do not last too long on paper as any crinkle is like breaking the circuit otherwise it would be a neat thing to keep around. 
​If any reader played with this please let us know what you thought in the comment section!

Ahhh good news for guitar players that can not be bothered with keyboards.
Android 6.01 is necessary though.
It is not out yet but there is the possibility to sign up to get to know when it is ready plus to know more about what to expect...
​http://android.jamstik.com/_

To read it go here:
http://www.thefuture.fm/blog/dj-technology/imaginando-releases-lk-ableton-live-midi-controller-iosandroid/

Playstore Info:

Since there are many request to play chromatically on Volca Sample, I have manage to create a simple app. You can connect your midi keyboard. I used Korg microKey 61. And play piano on your volca sample. Cost of the app is free. The setup is shown in the photo above.You will need
1. OTG cable $1 at amazon
2. usb to midi cable $24 at amazon
3. a 4 port usb hub. No need if you are using Korg microKeys as it already have 2 usb slot.

Alternatively you can buy the above accessories direct from me so that you can be sure they will work.

For the sample, you can use factory default 74 which is a electric piano sound on the volca sample. For me, I use my another app to record A4(440Hz) note from my piano and transfer to my volca sample. You can download it here at

Volca Sample Transfer - 
https://play.google.com/store/apps/details?id=com.appspot.volcasampletransfer

After you have a sample, you can tweat it on the volca sample. As you play on your keyboard, the sound will be based on your adjusted parameter!

The app auto configures your midi in and midi out devices. Alternatively, you can download the device button app "Midi Connect" to manually configure your midi.

Have fun!

Was going to write about this and forgot but saw this video and got reminded and god damn this will be leading to unexpected and of course obvious uses.
Midi keyboards and controllers for some direct playing and composing are the most obvious ones. But there will be more out of the box thinking when this comes into full swing. I am looking forward to when this is an actuality and it seems that the development is far ahead so it could be not too far of into the future.
One thing that suddenly makes smart watches interesting for music making is when this gets implemented. I have really been wondering what real uses smart watches are bringing to the table but now...
To read more:
http://mashable.com/2015/05/30/google-project-soli-analysis/?utm_cid=mash-com-Tw-main-link

Seems to be a must if being a DJ and using Traktor.

Play Store Info:

TKFX is a dedicated Traktor Pro Effects controller that features an X/Y pad. TKFX allows you to use Traktor effects in a completely new way. Select the effect you want to use, assign effect parameters to X/Y and Z axis, enabled the effect in the wished deck and touch the pad. The effect is automatically applied.TKFX is completely plug and play which means that you don’t need to waste countless hours in MIDI mapping. TKFX knows all about TRAKTOR PRO effects, just select the effect you want and all it’s parameters and functions will be automatically reflected on the respective buttons. TKFX supports effect unit operation in single or group mode.

Key features:

- X/Y pad
- Plug & Play
- Hold function
- BPM sync
- 4 effect units
- 4 memory banks
- Single or group mode
- Wireless or wired connection

In version 2.0 we have redesigned the app from the ground up to allow even more possibilities:

- Multi effect mode
- Assign multiple parameters to a single axis, including amount and on
- Link mode (to trigger four units at the same time with a single pad)
- USB wired connection
- Phone devices support

For detailed documentation please visit www.imaginando.pt

PRESS REVIEWS:

"TKFX is excellent fun to use and DJ effects respond especially well to this kind of real-time effect control." - Hollin Jones - AskAudio Magazine

"But wouldn’t it be nice to use a tablet for X/Y control of effects? TKFX does that, and nothing else – but it does it damned well." - Peter Kirn - Create Digital Music

"I recommend this app for anyone who likes to use the effects inside of Traktor, as it offers you an instant way to add and manipulate effects with ease" Alan Churchill - Traktor Tips

"In conjunction with a dedicated hardware controller, TKFX is a simple but effective solution for extended Traktor FX control." - Declan McGlynn - Resident Advisor

Another short review of the Ableton Live controller...
Go here to read it:
http://askaudiomag.com/articles/review-lk-ipad-and-android-ableton-live-midi-controller?utm_content=buffer32cc8&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

Here is a short review of the Ableton Live controller. It is made using a iPad but the Android version is the same...

To read the review go here:
http://www.residentadvisor.net/review-view.aspx?id=17074

The excellent website Create Digital Music wrote about the Ableton Live controller (that can be used for controlling other things to according to the article) formerly known as LIVKONTROL and they are pretty excited about it so if you use Ableton Live go read more here: 
http://createdigitalmusic.com/2015/03/lk-gives-ipad-android-tablet-easy-control-ableton-live/