Flowkey - The easiest way to learn piano with your iPhone or iPad
SeeMusic - Official Download for the SeeMusic App
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Synthesia - A fun way to learn how to play the piano.
Real Piano Teacher - Real Piano Teacher is a music learning application that lets you learn how to play the piano, organ, flute, Guitar, and many other instruments just in seven days.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.