Meet OpenRA[www.openra.net]: a freeware, open-source RTS project that recreates the classic Command & Conquer titles[en.wikipedia.org].

The game includes recreations of C&C (Tiberian Dawn), C&C: Red Alert, and Dune 2000. These are not intended to be perfect copies, but are rather meant to mix up classic gameplay of the originals with modern improvements such as but not limited to unit veterancy and the fog of war.

OpenRA offers a cross-platform multiplayer experience between Windows, OS X, and Linux, with a number of single-player missions to boot. And don't forget your favorite AI skirmish games!