Flowkey - The easiest way to learn piano with your iPhone or iPad
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Pianu - Learn to play the piano in your browser.
TuxGuitar - A Multitrack tablature editor and player.
Synthesia - A fun way to learn how to play the piano.
Touch Pianist - Play piano like a pro without playing piano.