Pianographique is an interesting shockwave flash site that allows you to assign sounds and animations to each key of the keyboard.  Once they keyboard has been mapped, you can create and record symphonies of sound and graphics by pressing or holding the keys.  The interface is a little cryptic at first and requires you to map every key before you can begin your composition, but once you get the hang of it, the results can be impressive.  You can also view symphonies that other users have recorded and posted for sharing.

http://www.pianographique.net/