Other freeware game projects

[Vector_Matt]

Tremulous Tremulous.net A free FPS with some RTS elements based on the quake3 engine.

[Nightmare]

Some games from this page are cross-platform, not only for Linux:

http://www.happypenguin.org

For example:
Freeciv
http://www.happypenguin.org/show?Freeciv
http://www.freeciv.org/

Freeciv Screenshot Freeciv is a multiplayer strategy game.

Freeciv is a free turn-based multiplayer strategy game, in which each player becomes the leader of a civilization, fighting to obtain the ultimate goal: To become the greatest civilization.

Players of Civilization II® by Microprose® should feel at home, since one aim of Freeciv is to have compatible rules.