Metronaut - An app to help you play classical music
Flowkey - The easiest way to learn piano with your iPhone or iPad
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
Flat - Online collaborative music score and tab editor, accessible from any device
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.