This time, however, I found the Spring Engine, which is a full, free, open source, 3D RTS engine. The game "Total Annihilation" was a popular RTS released in the late 90's by Cavedog that spawned a large following of fans. After Cavedog closed in 1999, the IP was bought by Atari. With no modern 3D remake being a possibility due to its closing, a fan-made 3D remake called Total Annihilation: Spring was made, and the authors of that game released their engine as an open source project.
I have not downloaded it or tried it out yet, but it looks very professional. The website is very sharp and commercial looking and there is a pretty long list of games, both indie and even proprietary, made with it (I believe the proprietary games will release their source mods to stay with the terms of the engine, but of course their content is their own, such as models, textures, music, and Lua-based gameplay scripts).
I would be interested to know if anyone here checks it out