Pongmechanik
Sone guy in Germany has built a mechanical version of the video game "Pong." The device, which is called "Pongmechanik" (how clever), "is an absolutely physical game. The game is realized electromechanically, and essentially consists of four elements:
A relay computer, the mechanical movement with collision detection, the display and the acoustic components."

As you can see from the photo, this thing is huge. It looks about the size of a foosball table with a couple of wired joysticks. The clear case was inspired by Konrad Zuse who created the first mechanical computer in 1935. Like Zuse's machines, the Pongmechanik showcases its interior parts.

The site, which may be hard to access because it was Slashdotted this morning, features a more detailed explanation of how the device works, photos and video clips.