SunVox / Virtual ANS / Pixitracker / Pixitracker 1 bit / Pixivisor / Nature Oscillator and two more synthesizers that I can not recall the name of and maybe are the least interesting ones.
What I do recommend though is to check out the latest one Nature Oscillator ( now for about a euro ) as I myself had dismissed it and was thinking that it would just be doing what Phonopaper could do in free mode but I was wrong and it does do some funky stuff with the sound. Yes it is based on the same sound engine as Virtual ANS and Phonopaper but it gives you some other controls and is recommended for anyone doing experimental or ambient music.
In either case if you miss any of the apps ( that are all close to my heart except maybe Pixitracker and the two synthesizers ) now is a good time to get them.