Europa Universalis!

Freeciv is an open-source, turn-based strategy game based on Civilization II. Inspired by the history of human civilization, the game commences in prehistory and your mission is to lead your tribe from the stone age to the space age...

C-evo is a freeware empire building game for Windows

FreeCol is a 4X video game, a clone of Sid Meier's Colonization. While remaining faithful to the original in terms of mechanics and gameplay, FreeCol sports a new set of redesigned graphics. Moreover, in addition to the classical Colonization rules, it features an additional ruleset that incorporates ideas that didn't make it to the final version of Meier's game, requests by fans and original concepts like new European players with new national bonuses.

It is the year 2077, for the first time in its history mankind has finally began the colonization of another planet. Will the pioneers from Earth succeed in colonizing their new home or will they fail to survive against the harsh and inhospitable conditions of the Red planet?