Isle of Tune
This is amazingly fun. Think of a musical SimCity. Isle of Tune is a music sequencer that let's you build a virtual roadway and line it with trees, lamp posts, houses and such that produce different sounds as you pass them in the car(s).
Phenomenal idea, though much like the person who pointed me at it, I had more fun checking out other peoples' creations - some of which are amazing - than building my own.
I so hope these people are going to turn this into an iPad application. The bomb if they implemented that way. Who wouldn't download it?