OpenRA

OpenRA is a Libre/Free Real Time Strategy game engine supporting early Westwood games like Command & Conquer and Command & Conquer: Red Alert.
OpenRA is a highly moddable RTS game engine, in fact both Red Alert and Command & Conquer are implemented purely as mods on it.
Red Alert
RA
The RA mod for OpenRA is a reimagining of Command & Conquer Red Alert.

It is not just a clone of the original but has improved and rebalanced gameplay. It has fully working multiplayer support for both internet and local play (via direct connect).
Command & Conquer
CNC
The CNC mod for OpenRA is a reimagining of the original Command & Conquer game. It has fully working multiplayer support for both internet and local play (via direct connect). A single player campaign is currently being worked on.