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