BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
Globulation is an innovative high quality real-time strategy which minimizes micro-management by automatically assigning tasks to the units. The player has to choose the number of units they want for the different tasks, and the units will do their best to satisfy the requests. This allows the player to manage more units and to focus on the strategy.
GalaxyHack allows you to design a fleet of spaceships which can then be tested in AI script based battles against fleets designed by other players. Though battles take place in real time, the strategy comes beforehand, both in writing short AI scripts in an easy-to-use proprietary scripting language, and also in the set up and selection of your forces.
In Warzone 2100, you command the forces of "The Project" in a battle to rebuild the world after mankind has almost been destroyed by nuclear missiles. The game offers campaign, tutorial, multi-player, and single-player skirmish modes. An extensive tech tree with over 400 different technologies, combined with the unit design system, allows for a wide variety of possible units and tactics.
Gigalomania is a 2D real time strategy game that is a clone of Mega-Lo-Mania. The gameplay consists of researching and developing new technology with which to conquer your enemies, from rocks and sticks to nuclear weapons and spaceships. You can advance through ten different ages, from the stone age to the future. There are 28 different maps to play through. The game is still under development, but is entirely playable in its current state.
Really Rather Good Battles In Space is a real time strategy game with fleets of spaceships. It is a single player real time strategy game following the fall of the great Artilian empire. It features a short but relatively polished single player campaign, pretty (albeit 2d) graphics, spaceships, and explosions.