Combat Arms!

Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux. The project is geared towards providing addictive arena shooter gameplay which is all spawned and driven by the community itself. Xonotic is a direct successor of Nexuiz with years of development between them, and it aims to become the best possible open-source FPS (first-person-shooter) of its kind.

Cube 2: Sauerbraten is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS.

Zandronum is a multiplayer oriented port, based off Skulltag, for Doom and Doom II by id Software.

Aion is a visually stunning MMORPG where your character wields devastating powers and sweeping wings to explore a celestial world.