Graphic source: Wikimedia Commons.
Freeciv is a multiplayer, turn-based strategy game for workstations and personal computers inspired by the commercial proprietary Sid Meier's Civilization series. The latest stable version of Freeciv is 2.1.3, released on January 23, 2008. The game's default settings are closest to Civilization 2, both in gameplay and graphics (including the units and the isometric grid).
Freeciv is included with most desktop Linux distributions, and continually rated as one of the best available free/open source games. Released under the GNU General Public License, Freeciv is free software.
Players take the role of a tribe leader in 4000 BC and have to guide their people through the centuries. Over time, new technologies are discovered, which allow the construction of new city buildings and the deployment of new units. Players can wage war on one another or form diplomatic relationships.
The game ends when one civilization has eradicated all others, accomplished the goal of space colonization, or at a certain deadline. If more than one civilization remains at the deadline, the player with the highest score wins. Points are awarded for the size of a civilization, its wealth, and cultural and scientific advances.
Freeciv can be downloaded for free here.