Nexuiz. It's kinda like quake, or it's a quake mod that made its way into a standalone. I'm not entirely sure.
Battlefield 1942, Battlefield 2, are both free.
Arena, Daggerfall, if you like old-school MSDOS RPGs
Starmade is also free I think? Unless that's just the beta.
SpaceEngine, it isn't so much a game, but a universe exploration thing. You can visit star systems and galaxies and take screenshots and the like. Everything that we actually have mapped out is accurately (as far as we can tell) represented in the starmap. Anything beyond is procedurally generated, including stars, nebulae, galaxies, etc.