0 A.D.

Next I'd like to mention 0 A.D., another of those projects so impressive it's almost hard to believe what it has achieved. 0 A.D. is a realtime strategy game, not unlike Age of Empires or Empire Earth, though with a fair share of "own" ideas that sets it apart from it's kind.
Focusing on ancient civilizations, 0 A.D. aims to be as historically correct as possible while still being an enjoyable strategy game. Every civilization is unique, and nine different ones are available at the time of writing. What impresses me most is that every one has it's own units and buildings, carefully crafted with lots of detail and brought together in a stunning game engine, drawing up a fantastic landscape. Watching ships reflect in the water waves or shadows fall from every tree or little building is a sight to behold.
Graphics isn't everything though, but thankfully 0 A.D. delivers on gameplay as well. Balance is good, AI is fun to play against (though I admit I am no expert) and there are loads of maps and options to try out. Best of all though, the game has multiplayer support, so you can play against other players, or team up against AI opponents. (A favorite of mine, co-operation simply is more fun.)

0 A.D. is a very complete game, but as most open projects it is always under development and many new features and improvements are released in a new version every few months. The current is called Alpha 13 Magadha and can be downloaded here for Linux, Windows and Mac OS. Of course, source is available there as well. As a fun bonus, the historical background of each release name can be read together with the release notes. Have a look, it's very interesting. :)

An example of the high level of detail put into each building in 0 A.D.