ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Hyperlax - Sit back. Relax. Explore faraway places. Enjoy the music.
Frets on Fire - Bonus content.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
StepMania - Rhythm game for Windows, Mac, and Linux.