DooM Legacy is an enhanced port of Doom, the classic first-person shooter from iD Software. It features high screen resolutions, an OpenGL renderer, fullscreen modes under X, client-server architecture for up to 32 players on IP-networks, Quake-style keybindings, a console, and more. It is available for Linux, DOS, and Win32.
Sauerbraten (a.k.a. Cube 2) is a free multiplayer/single player first person shooter, built as a major redesign of the Cube FPS. Much like the original Cube, the aim of this game is not necessarily to produce the most features and eye candy possible, but rather to allow map/geometry editing to be done dynamically in-game, to create fun gameplay and an elegant engine.
Legends is a fast-paced first-person-perspective online multiplayer game. It is designed to take advantage of the beautiful environments available from the Torque engine it is based on, while still offering the breakneck pacing and variety of styles available from such classics as Quake and Tribes. The addition of a jetpack adds a new dimension to mobility.
Open Quartz is a project to supply artwork to create a full game based around the Quake source code. The pak0 module contains content (maps, models, graphics and sounds) and Quake code which controls the behavior and rules of the game. There is also a playable version available that ships with the Darkplaces engine by LordHavoc. The final goal of Openquartz is to become a 100% replacement for the original Quake game data that runs in all the free Quake engines available at present.