Visual Beat - Interactive music video
Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
The Lucid Realm - Listen to music in 3D
StepMania - Rhythm game for Windows, Mac, and Linux.
Google Music Lab - Interactive experiments that use the Web Audio API
Osu - Osu is Music, Rhythm, Single and Multiplayer video game developed by Dean Herbert.