I've been playing around with this one a lot lately on my android phone.
(Caustic)
http://www.singlecellsoftware.com/
It's got a modular synth and an 8 bit synth and other nifty stuff. Lots of fun if you spend much time in Train, plane and/or bus stations. Punchy sound.
I downloaded the free program on my windows computer too just to hear how it sounds thru my monitors. Not too bad, although the program freezes everytime I exit and the latency is horrible but it's meant for a phone/tablet.
I know there are tons of other apps too so please list your favorites so I can try those as well.