Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Simply Piano - Fast and fun way to learn piano
StepMania - Rhythm game for Windows, Mac, and Linux.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Osu - Osu is Music, Rhythm, Single and Multiplayer video game developed by Dean Herbert.
Yousician - Learn to play music at home, at your own pace