"0 A.D. by Wildfire Games has been underway for around 4 years. The game is developed by a team of unpaid hobbyists from all over the world on a spare time basis. The game will be completely FREE to download once it is completed.
0 A.D. is an RTS game featuring 6 unique civilisations, and one of our aims is to portray these civilisations with historical accuracy. Players play against the computer in campaign mode or other players in multiplayer showdowns. Additionally, 0 A.D. comes with a powerful editor, and just about every aspect of the game - units, maps, campaigns, etc. - are completely moddable."
Quoted from GameDev.net