Freeciv is an Open Source empire-building strategy game inspired by the history of human civilization.

From the Stone Age to the Space Age - build up your empire with the goal of conquering all others to become the greatest civilization, or if you are unable to be the best subvert all others.

Freeciv Features:

Generally comparable with Civilization I & II
Up to 126 players
Artificial Intelligence (AI) computer-controlled players
Internet & LAN multiplayer (TCP/IP)
Support for a great number of platforms
Premade maps & scenarios
More than 50 playable units and 541 nations
Modpack support
Internationalization (i18n): translations for many languages available
In-game help system