Freedoom!

Red Eclipse is a fun-filled new take on the first person arena shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, with a general theme of agility in a variety of environments.

Warsow is a completely free fast-paced first-person shooter (FPS) for Windows, Linux and Mac OS X, set in a futuristic cartoonish world where rocketlauncher-wielding pigs and lasergun-carrying cyberpunks roam the streets.

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

OpenArena is a community-produced deathmatch FPS based on GPL idTech3 technology. There are many game types supported including Free For All, Capture The Flag, Domination, Overload, Harvester, and more. There are 17 unique player models to choose from and 12 weapons.

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

The Hammer of Thyrion project is a cross-platform source port effort of Raven Software's Hexen.

Take part of history’s greatest battle of Green VS. Tan. Fight amongst and against others online. Jump, shoot and roll your way into victory with fast paced objective based gameplay. Sometimes the greatest battles are fought by the smallest of soldiers.