A Video Game Made of Cardboard
Here's a video game (a sort of ultra-simplified Super Mario Bros) made using paper, some motorized rollers, a speaker, a knob, a cardboard box, and an intriguing little circuit board thingy called a Teagueduino. It's a little hard to explain what this is and why it's awesome, so just watch the video -- and stick around for the second half, in which the creators take it apart to show you how it works. Fun, but probably not much replayability.
If you want your own Teagueduino, they're hosting a Kickstarter project. For $160 you get your own electronics kit so you can make stuff like this! The source code for this program is available, along with configuration instructions.