OpenRA
OpenRA is an open-source real-time strategy game engine that recreates the classic Command & Conquer experience with modern platform support. It provides high-resolution display support, cross-platform online multiplayer, and active development. Three shipped mods: Tiberian Dawn (C&C), Red Alert, and Dune 2000. The project also hosts community-developed mods including Tiberian Sun in OpenRA and custom campaigns.
openra.net