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.
Regnum Online is a free fantasy massively multiplayer online role-playing game (MMORPG) game (although premium content is available for enhanced game play) which runs on both Windows and Linux and I must say it looks really good.
I recently bumped into a cool RTS (real time strategy) game for Linux called Hive Rise. Although the game is not open-source, it offers a free Linux download as a RUN file which can easily be installed on your Ubuntu box. To install it in Ubuntu, you only need to follow these steps:
World of Padman (or WoP for short) features a cartoon-style approach, with maps, characters, weapons and sounds all based upon the Padman animation series.
VDrift, TORCS and SuperTuxKart are 3 free, open source car racing games which run on Windows, Linux and MacOSX.
In this article I'll overview Cube 2: Sauerbraten, an open-source shooter running on Linux, which also provides a gaming engine for developing FPS games.
Following the article on configuring Urban Terror, a popular first-person shooter game based on the ioQuake3 engine, here is a guide for configuring the original ioQuake3 game, including customisation of cvars, using new maps, weapons and characters models, and more.
Playdeb, the games repository managed by the same people that run GetDeb reached Beta 2. So far they only have a repository for Ubuntu Jaunty.
Heroes of Newerth is an online, multi-player strategy game that is based upon DoTA (Defense of the Ancients) currently in closed beta, but S2 Games sent Phoronix Media 400 beta keys to giveaway and they say they can get even more.