Wildfire Games -- 0 A.D
0 A.D was released under the LGPL license some time ago, and it's definitely the BEST "Open Source" RTS available (both technically and visually).
Build instructions can be found right here .
I built it (under Linux) without any headaches in 20 minutes or so on my Core Duo @ 1,8ghz (2mb L2 cache) ... (totally impressed ... lol).
Now here are some screenies ...