Frets on Fire - Bonus content.
Flowkey - The easiest way to learn piano with your iPhone or iPad
Rocksmith+ - Rocksmith builds off of the fundamentals of music games like Rock Band and Guitar Hero by offering fundamental lessons in actually playing guitar.
Yousician - Learn to play music at home, at your own pace
Osu - Osu is Music, Rhythm, Single and Multiplayer video game developed by Dean Herbert.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.