Video: Pokemon. To catch them all, you have to have two of everything — two Game Boys, two (slightly different) copies of the same game and, if you're lucky, a friend to play with. Trading monsters was addictive, fun and included forced social interaction! Well, it used to: someone just built an Arduino so he could trade with himself.
I have no idea how hobby hacker Pepijin De Vos's Pokemon-Arduino actually works (something about EEPROM and a 5V SPI bus?), but operation seems dead simple: plug the jumble of electronics into a working Game Boy, fire up Pokemon Red or Blue, and trade a Pokemon. The digital monster then hangs out in the Arduino's memory until you swap cartridges and finish the trade with your other copy of the game.