http://www.youtube.com/watch?v=qyDfcyAkwe0

Circuit Synth is a tiny modular synthesizer for the iPhone and iPod touch:

Design your own instrument by wiring up simple modules.

Included are three kinds of oscillators (sine wave, square wave, and sawtooth wave), a noise generator, and modules for combining and modifying signals. Dabble in techniques like additive synthesis and frequency modulation just by choosing different arrangements and wirings.

Based on the video demo, it looks like Circuit Synth has a great interface but trades depth for ease of use.

If you’ve had a chance to try out Circle Synth, leave a comment with your thoughts!