* 0 AD is a historical real-time strategy open-source game.
* Red Eclipse is a single-player and multi-player first-person ego-shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, with a general theme of agility in a variety of environments
0 A.D. is an open-source, 3D real time strategy game with an ancient warfare theme, developed by Wildfire Games, and using the Pyrogenesis engine.
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.
Hive Rise is an RTS MMOG game in which your main goal is to build your equipment and make your soldiers win the war. This free, real time strategy game works on both Windows and Linux and is played online with thousands of other players, in short concluded matches and each player tries to achieve one of 12 possible objectives.