Flowkey - The easiest way to learn piano with your iPhone or iPad
Synthesia - A fun way to learn how to play the piano.
Yousician - Learn to play music at home, at your own pace
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Playground Sessions - Online piano learning game.