Technically it's 0 A.D., but that's a minor note. For those unfamiliar with the project, check out http://play0ad.com. It's a very mature codebase with over 10 years of development behind it. Everything is free as in freedom. One area they excel in that many other open-source games struggle with is in the quality of their models and art. I even know several other open source games which re-use the models.