Is there any good free open source games?

Gang Garrison 2

It all depends on what you are looking for, are you just looking for free games or is open source important? I ask because Dwarf Fortress is not an open source project.

0 A.D.
http://osgameclones.com/

FreeCiv, if you have someone to play with. It's not as good when playing solo.

Endgame: Singularity
http://www.emhsoft.com/singularity/

TripleA is like those giant WWII Axis and Allies board games.

Open Arena is just an open source version of Quake III arena with some graphics changes.

The Ur-Quan Masters.

I like Battle for Wesnoth, a very solid turn-based strategy game, and Hedgewars, a Worm-like game with... hedgehogs.

OpenTTD It's like the original TTD but better. And you won't need a copy of TTD to play OpenTTD.

Classic Spelunky

I don't think its open source, but try Aurora ( pentarch ), nicknamed the dwarf fortress of space.

open hexagon