The above video showcases a credit card-sized whatsit with a built-in screen, control pad and two buttons. It plays Tetris! If you're not convinced by now that we're either at or very near the pinnacle of human ingenuity, I'm not sure I'd ever be able to convince you otherwise and I'm not sure it's worth your time to keep reading this. We should amicably go our separate ways.
For the rest of you, this project is called Arduboy. It's about a millimeter and a half thick and apparently packs north of nine hours of battery life. Its creator, Kevin Bates, created the proof-of-concept you see in the above video and has plans to roll out a Kickstarter campaign to sell these things, complete with a website where people can share other types of software and games they create for Arduboy.
Bates writes on his site that he wants to use Kickstarter to raise $820 to cover licensing costs. I write here that he'll probably be able to raise that amount faster than he can clear the first level of Tetris. He'll also probably have to sell the cards without a game loaded onto them to avoid legal issues, though.
No word on how much a final version would cost, but you can visit Bates' website to read more about how the project came together, complete with photos of the Qdoba and REI gift cards he used to test some of the early builds.