EA will be releasing the C&C Tiberian Dawn and Red Alert source code under GPL3
I'm surprised that no one has mentioned Homeworld 1. Yeah it's not GPL, not even a proper open source license, it's also not well maintained like the id ones and Warzone 2100. But it really helps the mod community (and even helps the Remastered 10 years ago).EA says "this will be one of the first major RTS franchises to open source their source code under the GPL". Well it's not GPL, but Relic is famous for it's RTS titles like Dawn of War, Company of Heroes (and now it's developing Age of Empires IV). Homeworld is one of the best RTS games for me (only second to C&C).Sadly we can't get the source of Homeworld 2 (they even lost the source code of expansion pack), which the Remastered was built on.