Fretello - Duolingo for learning how to play guitar
Flowkey - The easiest way to learn piano with your iPhone or iPad
Justin Guitar - Over 750 completely free guitar lessons!
Yousician - Learn to play music at home, at your own pace
Uberchord Guitar - Get better on your guitar. Fast.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.