Definitely check out OpenRA, a modern open source and cross-platform reimplementation of the old Westwood engines, with support for CnC, Red Alert and Dune2000.

Definitely check out OpenRA, a modern open source and cross-platform reimplementation of the old Westwood engines, with support for CnC, Red Alert and Dune2000. There's a few changes to add a bit of balance but its still got the classic feel and there's an active multiplayer community online with regular tournaments and everything.