UberStrike is a free-to-play cross-platform online first-person shooter, available for both Mac and Windows PC. UberStrike is the world’s first browser-based MMOFPS. http://uberstrike.cmune.com/ (also available for Mac)

F.E.A.R. Combat is a free stand-alone multiplayer component of F.E.A.R., the horror themed first-person shooter. You have to register a free CD-Key to join F.E.A.R. Combat. System requirements: 1.7 GHz Pentium 4, 512MB RAM, 64MB GeForce 4 Ti / Radeon 9000. http://projectorigin.w F.E.A.R. Combat is a free stand-alone multiplayer component of F.E.A.R

Legions: Overdrive is a free Sci-Fi FPS based on cancelled project, Fallen Empire: Legions. Many gameplay elements in Legions: Overdrive are borrowed from the Tribes series. System requirements: 2.2 GHz Pentium 4, 512MB RAM, NVIDIA GeForce 6800 / 8400. http://www.legionsoverdrive.com/

Starsiege: Tribes is a multiplayer Sci-Fi first-person shooter with no single-player campaign. It was released for free to promote the release of Tribes: Vengeance, the upcoming sequel. System requirements: Pentium 166 MHz, 32MB RAM, 3D graphics accelerator. http://www.planettribes.com/

Red Alert: A Path Beyond is a first-person shooter based on classic real time strategy, Command & Conquer: Red Alert. This shooter is standalone and free to play. System requirements: 800 MHz CPU, 128MB RAM, 32MB DirectX 7 compatible video card. http://www.apathbeyond.com/

Tribes 2 is a sequel to Starsiege: Tribes, set in the year 3941. Tribes 2 is a completely free to download multiplayer first/third-person shooter that takes place in fictional Earthsiege universe. System requirements: Pentium II 500 MHz, 64MB RAM, 12MB video card. http://www.tribesnext.com/

Galacticwar is a free Sci-Fi first-person shooter that features both single-player and multiplayer modes. Galacticwar (GCW) will have single-player campaign and vehicles in the future. http://www.moddb.com/games/galacticwar

Xonotic is a free open source fast-paced first-person shooter that is direct successor of the Nexuiz Project. Xonotic features new physics, maps, player models and animations. System requirements: unknown, similar to Nexuiz. http://www.xonotic.org/ ( also a vailable for Linux and Mac)

Wolfteam is a fast-paced MMOFPS with many special features - like ability to transform into a werewolf. Wolfteam has large assortment of modern weaponry and 7 game modes. System requirements: 1.4 GHz Pentium 4, 512MB RAM, GeForce 5700 or higher. http://wolfteam.softnyx.net/ , http://wolfteam.aeriagames.com/

SCUD Storm is a freeware game that turns Command and Conquer: Generals - Zero Hour into a 3D first-person shooter. SCUD Storm is standalone, you don't need any game to play this. http://www.moddb.com/games/scud-storm