Call me a music geek, but this thing's amazing:
http://www.youtube.com/watch?v=0h-RhyopUmc
http://www.youtube.com/watch?v=MPG-LYoW27E
http://www.youtube.com/watch?v=WEDia3CFdfg
http://mtg.upf.edu/reactable/
(I'm thinking of trying to build one, but writing the code for the graphics and the synthesiser is intimidating... :P)