Rusty Russell wrote this really neat hack using a Wiimote, a projector, and two homemade infrared lights to play a modified version of pong. The two players draw lines on the wall with the lights and the ball bounces off them. Video of us playing at LCA Open Day here (taken by Paul Fenwick):
Unfortunately, the ball itself doesn’t show up very well in the video. You’ll notice there’s a bug where the line suddenly shoots across the screen in a straight line; Anthony Towns and I managed to find and fix the bug before Open Day was over. Open source development in action!