Thread dedicated to links to games that can be legally obtained for free. I'll start with a few of my favorites.
Marathon Trilogy (One the finest trilogies ever made, can't even tell you how far ahead of it's time this was. The engine was released under GPL and the game content was released free to use)
Command & Conquer Red Alert, Tiberian Dawn, and Tiberian Sun (TD re-released as freeware to celebrate 12th anniversary of C&C, RA re-released as freeware to promote RA3, TS re-released as freeware to promote C&C 4)
Nexuiz Classic (released under GPL, the fastest paced FPS I've ever played)
Urban Terror (stand alone version of a Q3A mod running on ioquake 3, beats the shit out of Counter-Strike and Tactical Ops)
Warzone 2100 (re-released under GPL after it failed commercially. A damn fine strategy game that got ignored when StarCraft and Command & Conquer were popular)
The Battle for Wesnoth (One of the best strategy games I've ever played, released under GPL)
Tribes 2 (re-released as freeware to promote it's sequel along with the original Starsiege: Tribes, community runs the servers now)