Swarmation: utterly thrilling single-pixel co-op game (Time Waster)
The only controls are the arrow keys. You control the blue pixel. Every other pixel is played by another human, and you can see their score by mouse-overing. That's about it, really.
I played Swarmation for about five minutes before I was suddenly overwhelmed with a feeling of pointlessness. It's fun -- and probably quite frustrating if you're a competitive type -- but it just wasn't for me.