0 A.D. is a free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare. In short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western civilizations, focusing on the years between 500 B.C. and 500 A.D. The project is highly ambitious, involving state-of-the-art 3D graphics, detailed artwork, sound, and a flexible and powerful custom-built game engine.
(Note: The game is still in an alpha stage and is nowhere near being fully complete yet, but please, try the game and help out the game's developers!)