Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Keezy Drummer - Make a beat in seconds
Osu - Osu is Music, Rhythm, Single and Multiplayer video game developed by Dean Herbert.
Sampulator - Make (and record) beats on your keyboard
StepMania - Rhythm game for Windows, Mac, and Linux.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.