From the announcement:
0 A.D. is a free, real-time strategy game currently under development by Wildfire Games. It's cross-platform, 3D and historically-based. Wildfire Games is switching from a closed development process to open source, making the game's code available as GPL and the art content available as CC-BY-SA.
More info can be found here.
This is a very nice development. If they keep/make the engine scriptable enough this could also become a platform for other kinds of games (RPGs are an obvious choice). Definitely one to keep an eye on.