Take 4096 LEDs, 64 RGB microcontrollers and four Atari joysticks, throw them (with some degree of accuracy) at a coffee table, and you might get this: a four-way Pong table. The whole top surface of the table has been turned into an LED array, the result of a two-year project by SparkFun Electronics.
Check out the video of the LED Pong coffee table after the cut
The tutorial explains each of the steps, with schematics and code available to download. Aside from a regular competitive game of Pong, SparkFun added in a collaborative game mode in which multiple balls are introduced and the four players must work together to keep them all in play.
They don't quite know how much the project cost overall, but add up the cost of 64 RGB microcontrollers and then factor in "a couple hundred hours of engineering and assembly". Like with many other super-cool geeky projects of this sort, I'm sure that if someone could release a relatively straightforward DIY kit they'd find plenty of buyers.