When the Nintendo Wii console came out, people were amazed that you could use the Wiimote controller to play games using physical movements, like swinging a sword, punching a boxer, or throwing a bowling ball. Well, a group of young engineers from Waterloo Labs apparently took it one step further by rigging up Half-Life (the flash version) so that you could play it with real guns. The "screen" is actually a slab of sheetrock covered with accelerometers so that the vibrations from your bullets (or rubber bands or rocks, etc) are detected as mouse clicks on the computer, thus killing the digital zombies and other creatures.
Unlike Mythbusters, they actually encourage viewers to copy their crazy experiments. Check out this article for a full list of hardware and software code used.