Flowkey - The easiest way to learn piano with your iPhone or iPad
Metronaut - An app to help you play classical music
Jameasy for Violin - Play rhythm game with a real violin. you can play with background music and real-time feedback
Violin by Trala - Violin by Trala is a music learning app that is designed specifically for Violin users.
alphaTab - alphaTab is a cross platform music notation and guitar tablature rendering library.
Simply Piano - Fast and fun way to learn piano