Are you (a) a budding Keith Emerson with (b) an iPhone and (c) US$1.99 burning a hole in your pocket?
Then Karn Evil 9 your way over to the iTunes Store and grab a copy of NESynth, which plops an old-school 8-bit synthesizer into an even older-school Nintendo Game system controller interface. The app gives you three pulse and one sawtooth waveform, plus a white noise generator, as your audio sources, as well as an arpeggiator, accelerometer-based pitch-bend, thirteen sampling effects, and an optional piano-keyboard mode. Better yet, you and a friend can play Mario World duets through peer-to-peer connectivity.
Here's a demo:
[Available from the App Store]