Using a small quantity of tritium (a radioactive isotope of hydrogen) and a pair of solar panels, Ian Charnas built a nuclear powered portable game system (a knock-off Game Boy) that is capable of playing Tetris. The tritium puts out an incredibly small amount of energy, so the system uses tiny but incredibly efficient batteries that were able to power the game for an hour after charging for two months.
If you're interested, Charnas is raffling the nuclear game system to benefit Chernobyl Children International. (via @pomeranian99)