Has anyone here played this game? Annex: Conquer the World is a free, open source RTS game that runs on the MegaGlest engine, and is apparently inspired by the Command & Conuqer series of games, as well as Earth 2140 (and sequels), StarCraft and Warzone 2100. There are four playable factions (with two more add-on factions available separately), several tilesets, campaign scenarios, skirmish and multipayer modes.
The same developer is working on another title with a space sci-fi setting, called Terra Centauri: Last Stand. A beta is available here. I haven't checked either game yet but from the screenshots and videos it seems pretty similar to later C&C titles (except with an Anime design style).