Splice Beat Maker - Make and share beats in your browser
Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Ramsophone - A generative art/music machine. (Be sure to refresh!)
Osu - Osu is Music, Rhythm, Single and Multiplayer video game developed by Dean Herbert.
Google Music Lab - Interactive experiments that use the Web Audio API
StepMania - Rhythm game for Windows, Mac, and Linux.