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

A small update and one thing that can be useful if you work with a lot of samples is that it now has the possibility to tag them making it easier to find the sound you are looking for.
As alway mentioning this app it costs very little and have huge potential. Specially for you that want to have a workstation that takes different time signatures and ease of creating 
poly rhythms to a level that is not seen in any of the other bigger apps.
The update:
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Improve scale modulation
Filter samples by tag
Fix guides that do not divide evenly into pattern
Fix step selection after step toggled
Fix display on high-resolution devices
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Play Store link:
Syntheogen

Wrote about this coming some time ago and now it is here since two weeks.
It seemed exciting first but can not see how I would use it for making music as the bpm is fixed. It does have its uses and it is fun to play around with though.
The first thing to do is recording samples directly into your device. The sample recorded is some seconds long and it seems that it decides a little bit randomly the playback speed of the sample at least on my device so it may be a bug? 
( If it is a bug it adds to the surprise and randomness in a good way in my opinion. ) 
After this you have to decide a shape for the sample recorded and that is what decides the basic rhythm. Then you can affect each side of the shapes with reverse. down or none. 
You also have three sizes for the forms which will affect the playback. 
Volume and pan gets controlled where depending on where the shape 
gets placed on the screen.
One other thing is that you can not save, 
so if you work out something it would have to be recorded directly.  
Feel in the end that it could be good to have on your device for the one in a hundred occasions where you would need something chaotic or sit and play with it and record a lot and cut out the best parts to use later preferably in a app that can timestretch samples.

For free and without publicity.
There is also a Webapp version to try so for lazy people that do not want to install first can try it out...

Play Store link:
Geometric Music
Webapp link:
http://geometricmusic.be/webapp/

So there is some apps that can be called workstations with synthesizers, sample capabilities, multitrack, sequencer and so on but there is only one that is for free and that is VuKNOB.
Finally will take some time with VuKNOB as it is now being actively developed with three nice upgrades the last half year or so. 
The last upgrade contains:
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* New JAM interface, multitouch support
* Automatic chords
* Basic arpeggiator
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It will be fun to check out this the days to come.
Play Store link:
VuKNOB

GoTo80 released a new album and there is a interview talking about the new album and how there is three hidden songs in the image above. 
Yes in the interview there is information on how to extract the songs and it is a funky thing.
Maybe we should all start to hide songs in similar manners and spread them out over net.
To read the interview:
http://www.theongaku.com/posts/goto80-on-the-hidden-secrets-of-files-in-space

So it just took one day for Tuna Knobs to get their Kickstarter goal fulfilled and now they have added some more offers for people that want to get into the action as early as possible.
There is a limitation on how many backers for each offer so if this looks interesting to you. Head over to their Kickstarter page and take a look. There will also be some other bonuses if they reach their new stretch goals as can get seen here.

For the ones that did not read my earlier post about Tuna Knobs it is rotary controls that can get attached on the screen and so it is possible to control rotaries of the app you are using.
So it means that there is many apps that it would not be useful for...
Which brings me to the next issue regarding developers.
They are asking for developers that have interest in making apps compatible.
Guess it would be a question of maybe making some rotaries in the app bigger or adding a new rotary control or two?

Tuna Knobs Kickstarter page:
https://www.kickstarter.com/projects/samuelverburg/tuna-knobs-stick-em-to-your-tablet-and-its-a-contr

It is very easy to use and what it does is randomizing a beat for you and you can see the rhythm laid out in notation. This you can change around with accents and changing the drums around plus more settings to adjust things to your liking.
This is excellent if you have a hard time programming drums as you can just copy the rhythm patterns in your sequencer when there is something that sounds good. 
As a bonus learning how to program rhythms just by copying the patterns would get fixed in your minds eye for future use.
Not to talk about using it for practicing instruments or to learn drums, hey there is even hand / foot and accent notation.
But there is one glaring omission and that is that it can only play one drum sound at a time...
So for the hi-hat patterns it is still necessary to think a little bit for yourself, the same with the kick and snare... and there is no time signatures except 4/4.
There is bpm though and it can still be of help as it can make the ground work easier for non players.  Hopefully it will be expanded upon as for me it could be a lot of help rhythmically.
For free and it is supposed to be publicity but there has been none on my device.

Play Store link:
Linear Grooves

Groove3 is a website dedicated to tutorials of music software and concepts.
Most of it is regarding plugins and Daw's but even if you are using just Android for your music making there is some tutorials that will apply. 
The ones I've seen and downloaded from before ( Reaper DAW tutorials ) have been very good and now after signing up yesterday will probably go through the one on the picture above as my drum programming skills are about 0.8%. 
So this offer is only for streaming the tutorials as they also sell the tutorials for download. Each tutorial is normally some hours in length divided up in bite sized videos.

So to get thirty days of streaming videos go here:
https://www.groove3.com/str/redeem_code.php
Write the code-  get30days
And fill in the information ( email, username etc ) and get a confirmation and that is it.
you can start to stream any tutorial.

Another great thing is that they have a Android app making it possible to stream the videos directly on your device.

Play Store link
Groove3 Player

The review is positive but seems to be a little bit uninformed but to their defense do not think that Android Authority's strength is reviewing musical applications.
To read it go here:
http://www.androidauthority.com/ufxloops-music-studio-review-401966/

Everything but the drums are made with the Bassline ( 303 emulation ) synthesizer.

So there is some new things in the update of course but maybe the most amazing is this: 

DJ App Edjing Raises $2.5M From Entrepreneur Venture And Deezer Founder 
Daniel Marhely

To read more about this go to:
http://techcrunch.com/2013/09/25/edjing
Guess there is real money to be made in mobile apps!

In either case the update is:
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- a new DJ social network 'edjing world' with your own DJ profile page to promote your talent and share your mixes to your fans
- More than 50 millions tracks
- A "Global search" feature: browse multiple music catalogues at once: local files, Deezer and SoundCloud
- A Curent Playlist to prepare your party sets in advance
- Music Library Optimizations and Redesign
- Flatter graphic design
- New ergonomics
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The most interesting part is the new DJ network as it promises unlimited uploads for free!
From their website:
"An unlimited Mix Cloud storage Upload as many mixes as you want! 
Find your mixes anywhere, anytime, on any device thanks to the new cloud storage system! 
Download your mixes from the cloud directly from the app in one click."

Better go their website and find out more:
http://www.edjing.com/edjing-new-version/

There is people commenting on the Play Store that the new version broke the app and did not want to post about this update first and spoke to a representative and they have now updated with bug fixes so hopefully everything is working nicely now.

Play Store link:
edjing Premium - DJ Mix studio
edjing - DJ Music Mixer Studio