Synthesia - A fun way to learn how to play the piano.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Playground Sessions - Online piano learning game.
Sampulator - Make (and record) beats on your keyboard
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
Ramsophone - A generative art/music machine. (Be sure to refresh!)