Circuit Synth is a tiny modular synthesizer for iPhone and iPod touch.
Here's what developer Michael Daines has to say about it:
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.
It wasn't that long ago that using virtual patchcords on a desktop computer was new!
Pricing and Availability
Circuit Synth is available for $3.99.