EA have just released the game client source code for Command and Conquer, both Red Alert and Tiberian Dawn versions. This code is used to generate a DLL that is then hosted in the game engine and will enable modders to create new content for the about to be released Command & Conquer Remastered.
The source code is available on GitHub under the GPLv3 license. This is not a complete release, it does not include game assets, nor does it ship with the game engine itself. Instead you can use this source to create game DLLs that are run in the engine. The code however is extremely well documented and is a nice peek behind the curtain of a successful commercial game.
You can learn more about the code release in the video below.