Tremulous
Tremulous is a free, open source game that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans. Players on both teams are able to build working structures in-game like an RTS. These structures provide many functions, the most important being spawning. The designated builders must ensure there are spawn structures or other players will not be able to rejoin the game after death. The overall objective behind Tremulous is to eliminate the opposing team. This is achieved by not only killing the opposing players but also removing their ability to respawn by destroying their spawn structures.
The Battle for Wesnoth
The Battle for Wesnoth is a free, turn-based strategy game with a fantasy theme. Fight a desperate battle to reclaim the throne of Wesnoth, or take hand in any number of other adventures.
Warzone 2100
Warzone 2100 is a 3D real-time strategy game, developed by Pumpkin Studios (Archived website) and published by Eidos-Interactive. In late 2004 Warzone was released by its copyright holders, Eidos-Interactive, to the public under the GNU General Public License and the Warzone 2100 Resurrection Project was born.
Although comparable to Earth2150 in many significant aspects, it does contain some features that are unique. These include various radar technologies, a greater focus on artillery and counter-battery technologies, more frequent in-game cinematic updates as gameplay progress, as well as a different vehicle design method.
Glest
Glest is a free 3D real time strategy game, available for several operating systems and that can be modified using XML and a set of tools.
The core game allows the player to take control of two completely different factions: Tech, which is mainly composed of warriors and mechanical devices, and Magic, that prefers mages and summoned creatures in the battlefield.