This is a video showing how, with a piece of sheetrock, a handful of accelerometers and a digital projector, you can play Half-Life by firing at the projected screen with a real gun. The first two minutes of the video explains how the system works, so if you don't care you can shoot(!) straight to 1:55 to check out the game play. Which, I should warn you, won't work with a regular LCD television. Or will it?!
UPDATE: It won't. And, uh-oh, looks like the Geek Squad is calling the cops.
Thanks to Alexandra, who actually knows the guys who made this and should introduce me so that I can shoot guns with them. pew pew!