Flowkey - The easiest way to learn piano with your iPhone or iPad
Pianu - Learn to play the piano in your browser.
Yousician - Learn to play music at home, at your own pace
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Joytunes - Learn music through mobile games
Synthesia - A fun way to learn how to play the piano.