Maestro
Maestro is a hack built at Music Hack Day Toronto. It allows you to 'conduct' your music by waving your iPhone around like a conductor waves their baton. You can speed up and slow down your music at will. Here's the demo:
[youtube http://www.youtube.com/watch?v=Q8NYaKTJZR0#t=0m46s]
The hack was created by Wen-Hao Lue and Peter Sobot.