"Tetris Jr." Tetris (415) 788-5550 Keychain $10
I don't know about you, but I find Tetris the most addictive of simple games. In case you've never seen it, you drop differently shaped sets of blocks onto a floor and attempt to fill a complete row for points.
One of the most appealing things about Tetris is it has no gender-tendency; unlike either male-specific shoot-'em-ups or, alternatively, a Barbie tea party, people acculturated either way seem to find Tetris very engaging.
It comes for all kinds of computers and electronic-game machines. And now, it comes in the most compact and portable form of all: the key-chain bauble.
Tetris Jr. costs $10 and makes a swell, battery-powered 1 1/2-by-2 1/4-inch key-chain attachment. The game is completely recognizeable, down to the optional music (in this version, the balalaika doesn't exactly constitute music, but you can turn it off).
It'll take experienced Tetris players a few games to adjust to the small controls, but it's surprisingly like the full-tilt game and even has a few extras I haven't seen before: special active blocks that do things like burrow through your pile.