Tonk Tanks is a really small, and fun game. All game play is on one screen, and the controls are simple.
You have arrow keys to move your tank around, and a button to shoot the other tanks. It's like one of those old atari 2600 tank games, but a more modern version. Once you die, you teleport to a different respawn positions around the map.
The game is quite playable single player - but the author is working on a networked multiplayer version. Another nice addition would be multiplayer on one machine - especially with joysticks or mice support (since keyboards are evil).
The tank AI seems different and varied enough, that I have not worked it out from playing it ten times or so. Usually my games only last about five minutes before I move onto something else. It's one of those games I can play for little while when I want a short break.
Tonk Tanks works well on linux, windows and Mac (and probably other platforms supported by python+pygame). There's a windows .exe available, otherwise you need to have python+pygame installed to play. I don't think it's packaged for any linux/*bsd distributions yet.