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.
Yousician - Learn to play music at home, at your own pace
Playground Sessions - Online piano learning game.
Jameasy for Violin - Play rhythm game with a real violin. you can play with background music and real-time feedback