Obviously based on the TB 303. For free and with a limited sequencer that seems to be planned to be expanded in a future paid version.

Google Play:
play.google.com/store/apps/details?id=com.rockrelay.synth.bassline

Square and Saw oscillator
Step sequencer
Midi support

The developer have an interesting set of Android apps and had this installed on an older now defunct mobile. So was going to install it again and now there is only two of the apps on the Google Play store and this one is not there anymore.

​Still listed/written about on their website as well as the other apps not on Google Play...

Will ask about it, can imagine that it is temporary... or maybe he decided to take away the free apps that came with ads?
The mysteries of developer and Google Play thickens...

It is for free with no publicity and that is always good, specially if looking for some acid sounds that is bundled together with some drum machines. 

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

Realtime emulation of a Roland TB303 synthesizer, TR808 and TR909 drum machines.
Draw in notes, adjust the sound parameters with knobs, auto create random patterns.
High end device required or it will sound like clicks and pops!

Play on your computer as a downloadable Java jar: 
http://bit.ly/2A73L0z

Contribute to the development:
http://github.com/raver1975/Acid

Please go ahead and share your creations with me, because I love acid grooves.

A couple hints:
There are three stacks, the Sequencer stack, the Drum stack, and the Knob stack. Use the little buttons to push to or restore from the stack, or click directly on pattern to discard the top layer.

When the "Free" button is activated, the knobs won't change or record, and the pattern doesn't advance to the next one in the song. You can kind of freestyle it.

Watch out for earburns.

Damian have been doing a lot of compositions using the RD4 lately which is nice as it is maybe not the best sounding app but on a tablet the most user friendly groovebox specially when using a tablet. Very easy to reach and manipulate most of the parameters.
But I would say that the best sounding 303 style groovebox is Audiod. It has a sharpness to its sound that cuts through in a good way that is missing from other Android 303's.
When it first came out it was pretty useless as it had some problems but with some updates it is a hidden secret. It has some deep sound shaping functions in one of the sound engines then the 303 plus two drum machines. It also sports one sequencer that works in its own special way etc.
I have written about it before but seeing this video felt the need to mention it again.
It works better on tablet as the it can become a little bit fiddly on a smaller screen. (With fiddly interfaces and small screens I suggest getting a pen...)
Anyway enjoy the video.

This is in my impression the one 303 that sounds the best and it also comes with two drum machines. In feature set it is very similar to Mikrosonic's RD4 and with a tablet RD4 is the more easy and fun one to use as a groovebox but as the sound is not as sharp as AUDIOID and as the interface as a groovebox is good enough on AUDIOID would / will be the one I would turn to if I needed a 303 workstation. Now it has one more synthesizer engine at it's disposal that is intertwined with a new sequencer type based on this hardware sequencer:

Here is the whole upgrade info:
__________________________________________________________________________________________________
- overwriting issue when recording
- performance issues (Audio DSP fully rewritten in RenderScript)
- linear/logarithm levels
- heavy UI overdraw
ADDED
- new machine MPL-185 (Metropolis-like sequencer)
- 2x chained FXs per machine
- reverse, ping-pong and brownian loop mode
- cloud save/load/delete (assuming a Google account is available)
- circular buffering
- Compressor FX
- a second oscillator per synth
- an LFO per synth (with tempo sync and gate sync)
- values display
_________________________________________________________________________________________________
So as is obvious there is a lot of exciting stuff that was not there before and was playing some with the new functions yesterday will continue tonight.
It is highly recommended and as the price is so low ( 1.99$ / 1.50€ ) do not see any reason not to get it and give it a go but there is a free version to try and lastly want to point out that there is separate smaller apps that can be bought with just the rhythm machine and one with just the 303 but have to say that you do not save much money and they are not updated.

Play Store link
AUDIOID

This is a great new addition to the Android music making applications if you are interested in the more acid / synthesizer aspect of music making, but can of course be used for more than acid type of sounds. It comes with three Synthesizers and one Drum track with the possibility to sequence and comes with some effects plus the possibility to record changes on the fly.
It is the same amount tracks, sound modules and being 303'ish in the sounds it creates it could be compared to Mikrosonics RD4. The biggest difference is that RD4 is much easier to use as a Groovebox ( if not the easiest to use Groovebox ) and RD4 comes with more effects, more possibilites and full midi integration but... 
Have not used PhoenixStudio much but my impression is it seems to have much clearer and better sound, making it interesting for people that would like something similar to RD4 but are more interested in the sound and less in the Groovebox aspect.

One thing that will hit you is the busy interface which has an explanation in that this application is maybe the oldest app you can get for Android!
Yes it was developed for PocketPC in 2001 so it was made for that format and for use with a Stylus so it was not made with a smartphone in mind. He is recommending to use a bigger screen like at least a Galaxy note or over but for me it works good on a 4.0 inch screen but as said above not a good idea or not the easiest app for live Groovebox action. In either case there is a full demo to use with only saving being disabled so you can test it and see if it is too fiddly for the device that you use.
Here is two of the demo songs that the applications comes with the problem that the audio files that I can upload play Wav files very badly sometimes and the latest mp3 converter that I tried does a bad job so if it is not playing the audio files properly download the demo version of the app and listen to the examples there, plus that it comes with more examples.

Playstore link:
PhoenixStudio

There is many 303 in combination with drum machines now for Android and here is another new one. Have not tried this one but if you are looking for more 303's this one is cheap and seems good enough. There is other ones with more tweakability but maybe the ease of use could be attractive for some without getting distracted by too many windows and options and it sounds good in the video. Not being expensive either is another plus.

Playstore link:
DRBox303

So since doing the 303 comparisons here at Musical Android there has been a slight obsession of that sound torturing mha soul. 
So this caught my attention on the Caustic forum when one person posted a link to this site with 303 patterns. So if you have one of the apps guess it would be nice to try them out?

Link to patterns:
http://www.acidvoice.com/tb_303_famous_patterns.php

It is made with Audiotool that is probably the best of the cloud based DAW's and of arguably the best 303 emulator for Android. Have gone physical in their first release of  a vinyl record of minimal techno by the artist Sumad.
They also sell t-shirts. Guess as they don't charge for the use of their DAW they are expanding to new commercial endeavours.

Links:

To buy the vinyl

To hear the artist

To use the DAW

Playstore link- Audiotool Sketch

And a video introducing their DAW and the Playstore video for the Application

Hey today have been recording these examples that follows-

It is not very fun to listen to because the idea is to hear the quality of sound, this have been slightly hard as there is not one neutral position that even sounds similar inbetween the applications. 
So the loop is as follows C note low, middle then up with slide and then middle with slide.
The sound samples start with a saw waveform in the neutral position that is the position pointing up on all of them- 
from there the frequency and resonance get turned upwards then downwards in a random fashion- after this there is a small break and the same thing gets done with the Square waveform.
Don't touch any other parameters and there is no effects used at all.  
To get a better feel for the different applications is maybe to just listen to the first part where the saw wave are used and the frequency go up together with the resonance.

One other thing that would be nice is if you as a reader are using any of these applications please write me some of your own observations to musicalandroid@yahoo.com or directly in the comments...

As mentioned in the overview post regarding Audioid this have for my devices a click on the beginning of each note and in the end there is switching inbetween the four different Audio qualities that you can choose from.

In the end there is playing with the pulse width control

In the end of the recording there is some tom-foolery going on with two more waveforms- 
one a version of the square waveform and one that has a real name that escapes me, 
but looks like a relaxed saw waveform...

This one only have a square waveform.

Okay so this is not an application but it is a hardware synth that came out in the beginning of the nineties it has a preset that is supposed to emulate an 303- as there is no hardware 303
in my possession was thinking that it could be interesting to at least hear some hardware too.
Have used the same methodology as with the applications.

So in the end what is the best ones- 
well counting out Audioid, Square one and the Novation Basstation to me sound wise would be Audiotool sketch- 
Then think that RD3 HD have a slightly bigger range in frequency and Resonance than Caustic, but the sound can be quite interesting with Caustic when you start to play with the controllers in conjunction with the pulsewidth. 
The other considerations is of course what else is included in the apps and what you are going to use it for.
If you are using Audiotool the bigger Internet DAW it feels like it would be weird not to have Audiotool Sketch as they are totally integrated with each other. Plus that if the device is crackling slightly as it does for me it is perfect to render from the Internet DAW with a cleaner sound.
Then is the question if you want to use it for composing or for live playing.
For live groove boxing the answer is RD3 HD as it is easy to tweak it has two 303's and ten drum kits (not tunable), effects that you can click on and off right on the side of the 303's and that you can also jump to, to control X-Y style and finally is that if you use ten inch tablet it can go portrait mode so you can see three modules at the same time to really switch things up fast and you can also record the whole performance right from the app. 
But for composing you would have to go Caustic for too many reasons to mention here and in the end the difference in sound from RD3 HD is not really too big...
and in Caustic you can decide note length and it has more twang to the slide.
Think in the end of all this will probably end up in doing some more experimentation but would reach for RD3 HD or Caustic but being more of a composer guy than a DJ live style guy the convenience is with Caustic.
Well now after all these chuppililupps we should take a video break and then go outside or something...

Felt that this worth to take a look at too, there even was a small tutorial series in the Computer Music Magazine how to make the 303 sound in SunVox.

This is from an album that been going strong here the last two weeks and in this song the 303 gets used pretty much;
Ceephax- Camelot Science.

Here is a link to a whole lot of recordings made with a real 303:
http://www.tb303.ch/148,0,tb-303-samples,index,0.html

Playstore links:
Caustic 2
RD3 HD
Audioid
Squareone
Audiotool Sketch

So this 303 is part of a bigger structure sound generator out of four different sound generators included in Caustic (synthesizer/sampler/303 emulation/and a drum machine)- Well caustic consists of the possibilty to use six sound engines complete with separate sequencers and effects.
Meaning that you could in practice use six 303's in Caustic. 
As Caustic is a mini Daw and not a groove box like the other applications it is not layed out to play in groovebox fashion, which is a disadvantage if you want to use it live, 
it is possible but less friendly in this aspect, 
but the advantage that it has is that you can record your automation over and over again 
til you get what you want. 
You will also sequence the 303 in a piano roll type of sequencer which is really nice as you can lay out a groove and then just move the notes around as you want which is not possible with the other applications so in this aspect it is easier to jam with.

The other difference that it has to the other emulations is that it has modulation built in to control either pulse width/Cut off or resonance. 
Then directly there on the engine itself it has four different types of distortion.
With pre/amount and post controls.
One last thing that none of the other 303 apps have is a pulse width control...

Okay so far so good. 
But there is always a but, soundwise it is lacking  some omph in the bass 
you can still get it to fill up the low end frequencies though.
Also feel it is a lack of clarity  in the resonance as well- 
you can kind of make up for it with some effects  (equalizer like some other effects in Caustic can be up to two insert effects per sound engine plus delay and reverb like send effect), still dont make up for it totally and feel that maybe of the ones tried so far and without having done a side by side comparison that maybe Audiotool Sketch has the advantage soundwise and number two RD3 HD.
We will see tomorrow...

Playstore link:
Caustic 2

So this have been really disappointing for me as trying this app 
on three different devices and even tried an older version, 
just can not get rid of a disturbing click in the beginning of every note. 
Maybe more annoying is that there is a lot of very positive reviews on the Playstore 
so I do not really now what is wrong with my devices 
or if people don't care about clicks in their sound- 
Well there is a free version so you can always try the app out first-- 
because if it is click free it sure is interesting- 
there is two versions one for 1€/$ with two 303's and one 
with the addition of a 808 style drum machine for 2 $/€.

There is a big difference in comparison with the other apps is that you have a pretty big pianoroll sequencer where you can click on and off notes and the up-down/accent/slide functions so to play notes live is much easier than with the other apps, 
what is not easier is with tuning/cut off/resonance/envelope mode/decay/accent controls 
that is much easier to control on the other apps-   
what you normally would like to use live. 
otherwise there is three oscillators- 
Saw/Pulse/Sine, five effects and some other really nice small touches for the pianoroll-
like slide pattern randomizer from simple to more complex, 
semitone up and down the sequences 
and also a really nice big equalizer that is meant to be used live.
One more thing it have is the possibility to switch sound quality from 11 kHz to 44 kHz

But also on the downside-
can only reach three of the effect units controls
and when jumping to mixer view the keyboard pops up in the way which is disturbing if you are doing things live.

Would definitively check out this one if you want a 303 as the smaller version is only a Euro/dollar or just try the free version first...and the sound, click free seems really nice.

There will be no sound test as it doesn't make sense and could not really find anything that could be used on the net well there is like two videos made with the old version that do not have the piano roll sequencer...

Playstore link:
Audioid 303

This is the "smallest" of the applications but it is for free so there is nothing to loose!

It doesn't really come with anything-
one 303 synth and one sequencer that you can program but just one pattern that can not be saved... at least the app will remember what you were doing when it gets reopened. 
So there is neither any way to save any sound files so this is strictly for recording into something else if it is to be usable in any music making.
It comes with buttons to program the C-Major scale that can get the notes sharpened by using the # symbol on the notes you want to affect why it is like this who knows? 
But it can be good to know as my first impression was that this was an accent as you have on the other 303 apps and on the original 303.
Otherwise it comes with all the usual parameters that you would expect on a 303, but also containing one delay, these parameters you can play live,
but the notes, octave up / down and slide functions are not tweakable in real time.

It is not exciting and it is not really suited for funtime groove-boxing 
even though the controllers are responsive enough it just sounds to weedy, 
it does not really have a lowend that would be useful.

One thing that it does have is a nice resonant filter that can selfoscillate and you can get some really nice textures using the resonance for interesting midrange frequency rhythms and it does have some sounds that at least me couldn't coax out of the other two 303's that have got tried so far. 
Can see myself reach for this once in awhile just for that specific resonance sound that you can get out of it. 

Frank Malm

Some sound samples

Playstore link:
Square One

So this is right of the bat better for groove boxing in comparison to Audiotool sketch, 
but maybe the sound a little bit more muffled which will be evident when they get compared side by side with the other 303 applications this weekend.
It comes with two 303's and one drum machine that includes ten classical drum machines. The 303's come with the original 303 controls but includes two more waveforms. 
Also included is five different effects ( distortion, reverb, delay, phaser, filter )  that you can map to four different effect slots two of which can combine two effects. 
The effects can be controlled X-Y style as well as having four on/off buttons right on the side of the 303's- 
So outside of the cut off resonance etc to control you can easily drop in the effects that you set up and control them in real time. 
The drums also have the possibility for one of the effects to be assigned individually.
If you use a tablet you have access to portrait mode where there is three windows visible and controllable at the same time. 
It also have the possibility to tap out the tempo so that is like in Audiotool good if you need to sync up with other sources live. 
It definitively feels stable enough for live tweaking which is cool as there is direct recording of your performance directly into your device. 
And finally multi touch so you can turn on and off many many things at the same time.


Mikrosonic the company sells two versions one for weaker devices and one version 
with some more functions but needing a stronger device and also with portrait mode for tablets. 
Two small critiques: 
One- you can't tune the drums.
Two- you can only program four 16 step patterns per sound generator.

In these sound examples there is both of the 303's used at the same time and if you spend just a little bit of time with this application you can build up some pretty nice grooves for using with other music or for live situations.

Playstore link:
RD3 HD

First up in this week of 303 comparison is Audiotool Sketch.

This 303 comes with a replica of the same controls you would find on a 303 original plus two drum machines one 808 clone and one 909 clone  also with the same controls and look of the originals. Just one effect a delay, and an overall groove function.
The controls that you want to tweak feels pretty responsive even if they could have been a little bit larger,
You can also tap out the tempo which is really useful if you want to sync in live with some other sound source.

One of the main points of the application is that to use it in conjunction with the bigger application that is the Web based DAW called surprise- Audiotool!
In the application you have the possibility to upload your work directly into the bigger Audiotool workspace on the net
Audiotool is very interesting and being for free you always have a pretty complete DAW as long as you have access to internet.The problem if you don't have internet and want to render to wav file is that it is not possible without first uploading your work into Audiotool. 
Or have the possibility to record into another device, and cut out the parts you want. 
 
To my ears it sounds pretty good but on the tablet that got used for the testing was crackling slightly and don't know if that is because of application or the tablet, so that would be a problem if you want to use the application in a live situation... but even with some crackling once in a while it is worth the buy if you like the sound as it is possible to sample just the good parts.

Recorded two short loops being tweaked so you get an idea of the sound but the end of the week there will be a more complete comparison inbetween the different 303 apps.
And am skipping the drums as it only considers the 303 emulations.

Playstore link:
Audiotool Sketch

Audiotool Homepage/DAW link:
http://www.audiotool.com/

So here is something that has been circling around in the idea sphere for awhile- 
Take the 303 clones that exist as applications and do a comparison. 
So the way that this will happen is that tomorrow and each day there will be one application represented like an overview/review and then the last day there will be a side by side comparison sound wise.
As this get started tomorrow and myself haven't done the side by side yet it will be as exciting for you as for me, because you are excited who wouldnt be!!!
But to kick things off take a hit of this documentary-