Metronaut - An app to help you play classical music
Synthesia - A fun way to learn how to play the piano.
Violin by Trala - Violin by Trala is a music learning app that is designed specifically for Violin users.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
Simply Piano - Fast and fun way to learn piano
Playground Sessions - Online piano learning game.