StepMania - Rhythm game for Windows, Mac, and Linux.
Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Osu - Osu is Music, Rhythm, Single and Multiplayer video game developed by Dean Herbert.
Frets on Fire - Bonus content.
McOsu - A free open-source circle-clicking rhythm game practice client for osu!
Audiosurf - Audiosurf is a music-adapting puzzle racer where you use your own music to create your own experience.