Nexuiz is one of the most popular shooter games which emerged after the open-sourcing of the Quake 3 engine, featuring a fast-paced game style and several game modes, like the popular DM or CTF.
Heroes of Newerth (or HoN) requires an invite to play, but it's developers created an HoN Holiday Open Registration form so you can sign-up for Heroes of Newerth for free - for the next 83 hours
Aquaria is an action-adventure single-player game by independent game company Bit Blot. The game is commercial and runs on Windows and Mac but as we don't usually write about commercial stuff, I will only present the Linux native version which is in open beta and is free until January (full version).
Commander Stalin is a free, open source real-time strategy based on Boswars game for Windows and Linux.
QuantZ, is a "unique puzzle action game" based on physics, where your goal will be to control a three-dimensional object positioned at the center of the screen. Balls that gravitate around can be eliminated by shooting them into balls of the same color.
Tremulous is a free, open source game for Windows, Linux and MacOSX (unofficial version for Mac OSX) based on the ioquake3 game engine, that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans.
PlayOnLinux is a free application that puts an easy to follow wizard style interface on top of WINE and contains some pre-installed scripts to help load a number of compatible applications and games.
D2X-XL is an OpenGL mod for the classic 3D game Descent 2. D2X-XL is based on D2X 0.2.6, has all bugs from D2X 0.2.6 fixed and has restored all features from the original D2_3dfx (official Voodoo graphics version). D2X-XL is free and runs on Windows, Linux and Mac OSX.
Smokin' Guns is a free, open source first person shooter game for Windows and Linux that takes you back to the times of sheriffs and bank robbers in the legendary Wild West.
Warzone 2100 is a free, real-time strategy and real-time tactics hybrid computer game for Windows, Linux and Mac OSX which offers campaign, tutorial, multi-player and single-player skirmish game modes, unit designer, unit experience system, and everything you would expect from a modern RTS game.
Just days after the release of Nexuiz 2.5.2 that brought many new features to this popular open-source game, Alien Arena 7.31 is out.
The developers of Nexuiz, a great free, open-source Windows, Linux and MacOSX game released version 2.5.2.
Glest is a free 3D real-time strategy game for Windows and Linux, where you control the armies of two different factions: Tech, which is mainly composed of warriors and mechanical devices, and Magic, that prefers mages and summoned creatures in the battlefield.
Blood Frontier is a free and open source single and multi-player first-person shooter game built on a complete remake of Cube Engine 2, which lends itself toward a balanced gameplay, completely at the control of map makers, while maintaining a general theme of tactics and low gravity.
Djl is an open-source (GPL licensed) game manager written in Python 2.5 for the GNU/Linux Operating Systems, inspired by Valve's Steam software for Windows for which anyone can submit games.
Urban Terror™ is a free-to-play multiplayer first person shooter game for for Windows, Linux and Mac OS X. It's a mod for Quake 3 Arena but it doesn't require it anymore, as it can run on ioquake3. Unfortunately Urban Terror is closed source, even though ioquake3 is under a GPL license, but that doesn't mean you can't play it.