Scummvm - Not a game but allows you to play tons of graphical adventures from Maniac Mansion to Myst.
http://scummvm.org/

OpenClonk - Adictive multiplayer 2D game
http://www.openclonk.org/

OpenLierox - Worms clone
http://www.openlierox.net/

Battle for Wesnoth - A good TBS/RPG game 
http://www.wesnoth.org/

Sauerbraten - Epic FPS with an *ingame* map editor. Yep. You read it right.
http://sauerbraten.org/

Open RA- Command and Conquer && Red Alert engine. You can download the game levels from with the game, no original CD install needed.

http://openra.res0l.net/

Vulture's Eye - The Nethack roguelike with Diablo 1 graphics - You maybe think it sucks but it's addictive. Also, permadeath

http://www.darkarts.co.za/vulture

FlightGear - A flight simulator with thousands of airports.
http://www.flightgear.org/

FreeCiv - Civilization I-II clone
http://freeciv.wikia.com/wiki/Main_Page

Lincity-NG - Classic Simcity-3000 clone
http://lincity-ng.berlios.de/wiki/index.php/Main_Page

http://sc2.sourceforge.net/ -  Basically , Star Control II. Explore, battle, and conquer the space.

http://vegastrike.sourceforge.net/ 3D space fight/conquest/traver   simulator.