Bebot

A $1.99 iPhone app: Bebot.

Touching the screen causes the robot to move and make sounds controlled by your movements. Play it like a musical instrument, or just have fun watching the robot and making sounds with your fingers.

Features 4-finger multitouch polyphony, multiple synthesis modes, user-definable presets and scales, tweakable synth settings and effects, and more!

Here's a demo:

(via Reality Carnival)