Original C&C and Red Alert via open source, so it works on Windows 7 and multiplayer lobbies work etc. Early stages but could be good soon.
http://open-ra.org/
OpenRA is an Open Source, Real Time Strategy game engine. Its primary focus is on creating an extendable platform to recreate games in the style of the early Command & Conquer games. It includes as official mods reimaginings of Command & Conquer and Command & Conquer: Red Alert.
We've tried to closely follow the spirit of the original games, while modernizing the interface, adding new units, rebalancing the gameplay and making use of today's powerful graphics processors. Our emphasis is on the multiplayer experience, though support for single-player campaigns are in progress.
Please note: OpenRA is currently at an alpha release stage. If you encounter a problem, please report it to the IRC channel or the bug-tracker.
OpenRA should run on any modern PC running Windows, Mac OS X or Linux. Requires Microsoft .NET Framework v3.5 SP1 or later; or an up-to-date version of Mono.
http://open-ra.org/