Freeciv

Freeciv is the open-source answer to a question many strategy fans have asked themselves: what if Civilization never stopped being updated? Inspired by the classic Civilization series, it lets players lead a civilization from humble beginnings through technological development, diplomacy, warfare and eventually the conquest of the planet, assuming your neighbors do not become irritated first. Its greatest strength is flexibility, with numerous rulesets, maps and multiplayer possibilities making it much deeper than its modest appearance suggests. Freeciv has been around for decades and remains one of the great examples of open-source strategy gaming. It is particularly appealing to players who enjoy turn-based empire building but would rather spend their money on pizza than on another strategy game.

“Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization.” - The Freeciv project

Review by Game Hippo:

“Freeciv is an impressive, detailed clone of civilization. It offers online play via servers but is not designed for single player games (although there are servers for this purpose and local games are possible if you make a server). In Freeciv, your goal is to gradually build up your civilization by learning new technology, by building improvements on your city, and by insuring that your citizens are happy. This game has a very steep learning curve, possibly with an exception for those accustomed to Civilization. This game may run a little slow on older machines. It allows users to connect to a special gaming server to make finding opponents very easy.“