What are you're favorite open-source games?

I like:

Frozen Bubble -- this is open-source crack!
PySol -- I love this game and find it indispensable
K/Gnome Mine -- I love the minesweeper-type games
SomeGnome -- I don't know why I like this game, maybe the challenge?
PenguinCommand -- I have loved missile-command-type games since my Atari 2600!
KBattleship -- Another type of game I've loved for many years

So what do you like? I don't really care about the commercial games, so let's try to remember only open-source games. Thanks.