0 A.D. is a free, open-source, cross-platform real-time strategy video game (RTS) and is it still under development by Wildfire Games. The game spans from 500 B.C. to 1 B.C. in its first part and 1 A.D. to 500 A.D. in the second part.
So, in short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western Civilizations between the years of 500 B.C. and 500 A.D. In the game, you build your base and train them for war.
To make the game enjoyable, it also features other elements such as combat and technology research. It involves excellent 3D graphics, detailed artwork, sound and a flexible and powerful custom-built game engine.
Additionally, it offers 12 different civilizations, each of which represented in the best way. Similar to Age of Empires, you advance from Village phase to City Phase and the town phase in the mid. As you advance in phases, new units, buildings, and technologies get unlocked.
More interestingly, the game is cross-platform playable on Windows, OS X, Linux, and OpenBSD. The game is entirely free and is still in an Alpha platform so please try out the game and help out its developers.