I officially now have an app on the Android market. It is called the melodotron.
The Melodotron turns an Android phone into a dynamic, user interactive, musical instrument. The sounds that it produces are completely dynamic and synthetic (no sampling or anything). As of right now (Version 0.5.2 at the time of this posting), the Melodotron can play notes on 4 octaves, across 3 different types of wavetypes (sine, sawtooth, square), all with an orientation activated tremolo effect. I expect to add more wavetypes, effects, and other improvements as regularly and often as possible. The source code is entirely free.
Check out the Melodtron's main page. The program is completely open source. Better yet, check it out on the Android app market.