http://www.wildfiregames.com/0ad/
Really promising game
(under heavy development). Probably the most promising community game project I've seen, certainly the most promising RTS project.
Check out the website, you'll be amazed. Graphics is disturbingly good (screenshots, concept art... not so much great technology as great style). It's meant to be heavily based on history, except for areas where it would harm gameplay.
Oh, it will be free, possibly even opensource. The authors promise lots of modding potential.
Six nations: Hellenes, Romans, Iberians, Persians, Carthaginians, Celts.