Member the good old days on Android when every second music app that came was a Theremin based app? And now there is a new one from the late 2016 that looks good (have not tried it as my Google Play is hooked up to Paypal and that account is empty at the moment Wink Wink...).
Some effects and other parameters to adjust.

The developer is also hold accountable for a web app called BlokDust.

BlokDust:
​https://blokdust.com/

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

This is a multitouch synth instrument. Play it by touching and swiping the screen, changing the waveforms and sliding the effects to create anything from traditional sci-fi theremin sounds to deep scuzzy soundscapes. Use the looper controls to record, loop and overdub layers. Then if you wish to save or share your recording, hit the download button to send your wav file to other compatible apps like Messages, Email, Dropbox etc. 

The original Theremin went live in 2014 on the web at femurdesign.com/theremin. Over a million hits later and hundreds of requests for a record button, I have redesigned it. Now the Theremin is ready for you to experiment with. 

Happy playing!

-- SPECIFICATIONS --

• Record, loop and overdub options.
• Multitouch allows up to 8 tones to be played simultaneously with your fingers
• 3 sliders: delay, feedback and scuzz
• 4 waveforms: sine, square, triangle and saw
• No internet needed, works both online and offline
• Download the loop as a .wav file
• Loops can be shared with other apps like Mail, Message and more.
• Optimized for mobile, tablet and large screen

-- REVIEWS --

Gizmodo "Just mess around! Twiddle those faders! Swipe and click with reckless abandon! Turn the volume all the way up, give the dog a panic fit, and make your neighbours think the street's being invaded by giant cabbage people from planet Zok. Trust me, this will never get old.”

Dangerous Minds “New online theremin simulator kind of sounds cooler than the real thing. Check it out and annoy your co-workers.”

Mashable “Messing around with this virtual theremin is completely addicting”

-- CONTACT --

To find out about my other musical app projects, please go to femurdesign.com and share your loops with me on Facebook and Twitter.

Facebook: facebook.com/femurdesign
Twitter: @femurdesign

For any queries and support contact me here:
femurdesign.com/support

Greatness and holy easy DIY!
There is not more information on the website but here is a link anyway if there is a desire to look what more Yuri is up to.
http://yurisuzuki.com/works/three-radio-theremin/

The developer of Moon Synth and Psycho Flute ( both recommendable ) was recommending this and it was a good thing as even it is limited soundwise it sounds very good.
Actually it just contains three sounds but I am already in love with one of the sounds 
( an olden time electronic choir sound ) and see me using this as an instrument just for 
that sound.
It functions as most of the other Theremin apps and has the possibilities to adjust Octave, Scale, Key, Size and choose one of the three sounds. It is possible that many would not really like any of the sounds but for me like said am in love with one of them and being multi touch and with the possibility to directly affect velocity it can become a good thing to add to your arsenal of instruments.
Plus it is for free and no publicity!
EtherSurface

Did mention this one before but it seems that it did not get enough attention since then and so here is another slightly larger post...

It is listed as Alpha version but can not find any bugs and suchlike and am curious where this is going when it is finished as it has good sound and a decent amount of parameters already.
Can be used a theremin or with keyboard. Like that the theremin pad has the piano keys visible to know where you are notewise. 
A great thing with the app used as a theremin is that you have a lot more control over the sound which is not the case with other theremin apps.
It is possible to use on a smaller screen but definitely advantageous to use on a tablet.
Yes the best of all it is for free and without pesky publicity!

Here is the user manual and also a easy way to see what the synthesizer contains:
__________________________________________________________________________________________________

The program offers two operation modes:
I. Creating Sound Mode.
II. Performing Music Mode.

I. Creating Sound Mode
1. Two equal buttons «∞» («Performing Music Mode»)
Set the program to a mode designed just to perform music with the sound set up in Creating Sound Mode.

2. «Preset Manager»
It is the unit to control previously created and saved sounds (presets). It contains:

- combo box with names of presets currently available to load. 
- button «S» («Save») – saves current preset with the same name. 
- button «S?» («Save as…») – saves current preset settings with new auto-generated (for now) name.
- button «R» («Rename») (temporary inactive).

3. Button «Simple/Advanced Mode» (temporary inactive)
It is for altering program modes between so called «Simple» and «Advanced».

4. «Tunable Feature Selector»
When pressed opens a list of modules available to tune.
In this version available modules are: «Patch», «Delay», …(to be continued).

5. Module «Patch»
It is to create a sound timbre.
A timbre is synthesized by mixing signals of four first harmonics with a base tone signal and a signal of Noise Generator. Each of them (Harmonics, Base Tone and Noise Generator) has it’s own ADSR-curve that describes amplitude changing.

Wave shapes: 
0 - no signal, harmonic is off (note: base tone can’t be turned off) 
1 - sine 
2 - triangle 
3 - saw 
4 - square 

Parameters of ADSR-curve: 
«Attack Time» - a time since a moment at user (player) touched the screen (KeyPad) till harmonic reaches amplitude maximum level. 
«Decay Time» - a time since the end of «Attack Time» till harmonic reaches of amplitude level defined «Sustain Level» value. 
«Sustain Level» - a level of amplitude (relative to maximum) that harmonic reaches at the end of decay phase. And it lasts until player released the screen. 
«Sustain Fade» - amount (coefficient) of signal fading out during sustain phase. A value of «100» means that signal won’t fade out until released phase. 
«Release Time» - a time since player released the screen till harmonic fades out completely. 

6. Module «Delay» 
Adds echo to sound played. It contains two equal sets of controls: one per channel.

- «Left On/Off» & «Right On/Off» buttons – turn effect on/off in left and right output channels respectively. 
- «Delay Time» sliders – set the amount of milliseconds between repetitions of echoes. 
- «Feedback» sliders – regulate the amount of effect’s output signal that will be redirected to input again.

7. Button «RND» 
(experimental)This button is that drawn in random way. 
When it pressed the program generates a random set of «Patch» module settings that can result in a quite interesting sound appeared. Attention! Settings made before are deleted without warning! (For now.)

8. Slider «Pan»
Allows us control a virtual horizontal position of «dry» sound (not touched by effects, such as Delay) between left and right speakers (if we have such).

9. Slider «Master Volume»
Allows us control an output level of final mixed signal (after all effects applied).Warning! Too much amount of this value can cause distortion of sound!

10. Section «KeyPad» 
Contains settings for sound production area:

- «Amt Keys» (= «Amount of Keys») – amount of musical semitones emplaced at KeyPad working area. Varies from 12 to 88. 
- «1st Key» (= «First Key on screen») – the first (lowest) musical tone of sound production area. 
- «IM/DM» button (= «Indiscrete keys Mode/Discrete keys Mode») – toggles pitch change modes. Pitch can be changed in stepless (IM) or in stepped manner (DM), when steps correspond to the natural harmonic series. 
- «BW/R» button (= «Black'n'White/Rainbow») – toggles painting modes of sound production area. Notes can be painted as black’n’white piano keys (blurred or sharpen according to «IM/DM» setting). Or it could be painted in rainbow shades (when «red» edge is for lowest tones, and purple is for… guess what…) 
«H on/H off» button (= «Highlight on/Highlight off») - turns on/off the mode of highlighting touches in sound production area. In Performing Music Mode when touched frequency is equal to one from the natural harmonic series corresponding keys of small piano keyboards are highlighted with green color (yellow - in «almost equal» case when «IM» is active).

11. Small KeyPad («keyboard», sound production area)
A rectangle field one must touch to make device generate a sound of distinct pitch and volume.

***************************************************
II. Performing Music Mode
1. Sound Production Area (KeyPad, «Keyboard») 
It is a full screen variant of a Small KeyPad. 
The difference is two small piano keyboards, above and beyond KeyPad itself. They can help to look after pitches player touch.
__________________________________________________________________________________________________
And finally some videos that does not really show the application at its best...
but like said it is for free so just download and take it for a ride.

Playstore link:
TouchVox

It is for free and it has a nice set of controls for the sound.
Plus it does not have any publicity.
There is a lot of Theremin apps but this one is a step above most of them, 
so why not try it out?

Play Store link:
TouchVox

It is not much of an update but it is a good reason to remind people of this nice synthesizer application that have been around for a long time now and it is a good thing that it is still being updated and still being for free and without publicity.
The new preset theremin is nice and emulates a theremin pretty good.
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- Remove speaker pops when app is in background
- Add new THEREMIN instrument (the future is now!)
- Other minor design touchups.
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Play Store link:
Saucillator

The application was developed by a band called Special Effects Department to be used as an instrument in their own music. There is some other apps that people made for themselves to use in their music and then released normally for free which is interesting as it also indicates that it is applications that are thought out with functionality in mind for making their own music. The video is a review / walkthrough of the app and it is not more to add than that. Tried it for a bit and it seems solid and useful.
As you can see I also added one album from their Bandcamp page. 
Guess that they have used some of the application for making the music?

Playstore link:
S.A.M.M.I.

It is small in functions but effective.
There is a synthesizer part that is limited. Then there's the interesting part. The possibility to record a two second sample and manipulate with the effects and by playing it on the touchboard / theremin application stylelelee. 
The idea of the application is to make the sound gritty / rusty / junky.
That's why the rusty interface and it makes it look interesting, different and peaked my curiosity.
Perfect for using for sound effects / instrument recordings on top of something else.
Some crazy sounding vocal snippets maybe or a short instrumental riff. 
As limited as it is on features do not let it fool you as it can be very useful in the right place or even finding an original sounding main riff for a large musical piece.
I hope that the developer do expand on this application or gets going on something more advanced in the form of a new creation.

Playstore link:
Skrot

If you want to know which of all "theremin" applications is the most advanced it is not a standalone application it is one of the many facets of SunVox hidden in one of the menus.
Always felt that just the Theremin in SunVox is worth more than the price of SunVox. 
It is hard to find a more versatile instrument.
What you can do with this instrument is to be able to take any of the modules plus effects and record them in real time polyphonically with pitch bends / slides / volume  control into the sequencer / song that you are working on. Most amazing to me is that it works so good using samples... always fascinated that they do not glitch- using the same samples in the sequencer and they get cut off you get the glitch but using the Theremin no.
For you that do not know and have a feeling that even many SunVox users do not know or have explored the possibilities of it, here is your chance to explore, record and present your findings / recordings to the world.
Because one SunVox user that is going under the name of Nopcorp has taken on the responsibility to organize a contest using only the Theremin of SunVox.
So there is no prize but there would be exposure to your music through Nopcorp and surely Mr. Nightradio will post some of it! 
Finally the contributions will be posted here at Musical Android during the contest and will also post the links to the music of the contributors which means that it would have been seen the day of the posting by around 700-1000 people plus the views coming later. 
There would also be a posting in the end with all the contributions for people to vote on their favorite.So it adds up to a humongous bunch of entities.
The winner would also have  a more permanent posting under the Contest banner. So in this day and age when it is hard to get people to hear your music think that this is a pretty good incentive to participate!
Lets not forget that if you are like me that only have used the Theremin occasionally or never at all will have gotten the bug and will probably incorporate more live recordings into whatever song you are working on in SunVox which is of course excellent in this day and age of static sequencing. 
Or to start to use it as a standalone instrument!!!
Okay lets get to the rules and conditions.

Directly from Nopcorp-
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RULES: 

MUST be created on a touch device. It can be a lo-tech such as a Pocket PC, 
or a hi-tech device such as Tablet or Ipad.

The CONTEST will be divided into THREE categories:

1) lo-tech devices competing against each other.
2) hi-tech devices competing against each other.
3) all devices competing together!

SUNVOX REQUIREMENTS:

- a TOUCH device such as a tablet, Ipad, Android, or pocket Pc.
- A sunvox file that includes at least one GEN; but you can have a maximum of four GENS.
- Automation and Programming is allowed, though be creative and use your touch device to create with.
- Use as many effects as you like.
- Use as many patterns and channels necessary.
- There is no minimum or maximum length of composition. 
- Drumsynth not allowed. 
- No samplers.

WHAT and WHERE TO ENTER:

Entries can either be a download link, or a zip/rar emailed to onlycrumbs@gmail.com.
Send both, a sunvox file, and an exported version (minimum 320kbps.) 

DATES TO REMEMBER:

- All entries DUE October 25th, no later than the 26th. 
- On the 27th a link with, ALL contestants entries, will be provided for download. 
- One vote per category. You cannot vote on your own entry. Votes must be emailed by October 30th.

AND THE WINNER IS...

Winners will be announced October 31st. Halloween! Perfect day for a Theremin.  
Remember, this competition is about the Touch Theremin and its amazing capabilities as a performance tool, but also how it can be used within Sunvox.

If you have any comments, questions or suggestions, please share.
Good Luck!
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Playstore link:
SunVox

Link to the Forum thread:
http://www.warmplace.ru/forum/viewtopic.php?f=10&p=9123#p9123

Remember that here it is the Computer version of SunVox being used which has the disadvantage of not being able to use the Theremin polyphonically as you would have on your device!

Just posting this because I am a sucker for new ways to interact with the devices...
It does not make any sense as it is much easier to just use your fingers as in many of the other Theremin applications... But was bobbing and weaving my face in front of my telephone yesterday evening and that kept me entertained for about five minutes!
Ohh yes it is for free- so for this entertainment you do not have to pay anything.

Also do not like to make fun of anyone but if you write the following grandiose description you kind of have to blame yourself. 

From the Playstore description:

"This is to the stylophone and theremin what the electric guitar was to the lute.
In the future all music will be made this way.
Do you want to be part of the future?"

Playstore link:
ThereminCam