Some more freeware games. I've only tried Freedroid and Wesnoth. I like em both. Freedroid is an orgie in nostalgia from my C64 (and Amiga) days. The other games seem ok, but I haven't tried them so I can't say. Also the two last ones are either only for the Unix based platforms or not finished (playable?).
Freedroid (remake of Paradroid)
http://freedroid.sourceforge.net/
Marathon (freeware release)
http://source.bungie.org/
SCOURGE
http://scourge.sourceforge.net/
Advanced Strategic Command
http://www.asc-hq.org/
Tower Toppler
http://toppler.sourceforge.net/
Battle for Wesnoth
http://www.wesnoth.org/
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Wormux (Linux clone of Worms)
http://www.wormux.org/en/
U.F.O. - Alien Invasion (not complete, but seems promising)
http://www.ufoai.net/