0 A.D.

"Wildfire Games, an international group of volunteer game developers, proudly announces the release of "0 A.D. Alpha 11 Kronos", the eleventh alpha version of 0 A.D., a free, open-source game of ancient warfare. This alpha features new Celtic factions, many improvements to the graphics, functional gates, a new sound system, improved AI and more!" - from the Wildfire Games web site.

I've been trying out 0 A.D., very good-looking game. As it's an alpha, there are a few bugs and missing features, but it's very playable. It's a real-time strategy game along the lines of Age of Empires/Age of Kings, with beautiful 3-D graphics, built on its own unique game engine. Campaigns haven't been implemented yet, but there are a variety of scenarios that can be played, or you can play on a random map. Unfortunately, lag gets to be very bad if you play with more than 2 factions on the map; I would recommend sticking to the "2-player" scenarios until that's fixed.

Check it out, it's at http://wildfiregames.com/0ad/