Built-In Mods
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

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).
It can be run with the provided shortcuts on Windows and OS X or the command line option "Game.Mods=ra".
Command & Conquer

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.
It can be run with the provided shortcuts on Windows and OS X or the command line option "Game.Mods=cnc".
OpenRA