0 A.D. (pronounced "zero ey-dee") is a free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare. In short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western civilizations, focusing on the years between 500 B.C. and 500 A.D.

0 A.D. is an open-source, 3D real time strategy game based with an ancient warfare theme, developed by Wildfire Games, and using the Pyrogenesis engine. 0 A.D. started originally as a mod concept for Age of Empires II, but in 2009 Wildfire Games licensed the code under the GPL and announced it will be open-source, available for Linux, Windows and Mac OS X.

Windfire Games has released the first alpha version of its cross-platform, open source real-time strategy game 0 A.D.

Based upon the Cube engine, AssaultCube is a fast paced, first-person shooter, (partially) open source, cross platform game. Formerly called ActionCube, this game can be played both online with hundreds of other players from around the world or in a single player mode.

While there are many open-source games available (just see the recent discussion about the most advanced GPL-ed FPS), most of them are based upon the ioquake3 game engine that in turn is based upon id Software's open-source id Tech 3 engine. There's also games like Nexuiz that use the DarkPlaces engine and then also Warsow that uses QFusion.

Gregory Ray, lead organizer of LivingArcana.com announced today that he has hired the Brazilian programmer, Gutemberg Ribeiro, of “Conceptus” an open source game engine, to port his engine over to Apple’s iPhone platform. The port of the game engine will remain open source and will be available under LGPL license to allow true 3D game development on the iPhone.

MegaGlest 3.9.0 was released a few days ago. The latest release of this free (as in liberty and in beer) multi-player real-time strategy game brings lots of new game assets such as nicer units models, animated game environments (tilesets) and several new maps. But most importantly cross-platform network games are now stable (no more "out of synch" errors between Linux and Windows).

Classifying Open Source production models is not an academic curiosity, as result from recent conversations on how the development model affects at large the software life-cycle and, more important, the business strategy.

Freeciv is a turn-based strategy game, with some real-time elements, in which each player is the leader of a civilization, fighting to obtain the ultimate goals of a nation: The extinction of all other nations, being the first nation to plant a colony in another star system, or survival to the end of the game.