Woah. I just stayed up till 2am (as you can see in the timestamp) playing this new game called Synaesthete. Definately worth the almost 90MB download. Think like a combination of a music game like Dance Dance Revolution and an action game kinda like Crimson Land.

Set in an amazingly colorful 3D dreamscape environment you run around avoiding/killing monsters and fighting the occasional boss battle with your lasers. Except to shoot lasers you have to push buttons in time to the music. And the music evolves and changes with you as you go through the levels and stuff. Its neat, and free to play.