Simply Piano - Fast and fun way to learn piano
tonestro - Practice & improve on your wind instrument while having fun
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Violin by Trala - Violin by Trala is a music learning app that is designed specifically for Violin users.
Yousician - Learn to play music at home, at your own pace
Synthesia - A fun way to learn how to play the piano.