The Civilization series has inspired several unofficial Civ-like games.

The most notable ones are probably C-evo, Freeciv, and FreeCol. The first two resemble Civilization II, while FreeCol is a remake of the original Sid Meier's Colonization.

Those three are all free to download and play, and there are some options for two or more human players to compete. There is no sign that the developers plan to stop making improvements.

The term "unofficial" should probably include the commercial game Call to Power II, because it was produced without any license from any company associated with Sid Meier after the producer had had a license to produce one game using the name; that was Civilization: Call to Power.

Civilization Series
Game Expansion packs
Civilization None
Civ II Conflicts in Civilization  •  Fantastic Worlds  •  Test of Time
Civ III Play the World  • Conquests
Civ IV Warlords  •  Beyond the Sword  •  Colonization (Total conversion)
Civ V Gods & Kings  •  Brave New World
Beyond Earth Rising Tide
Civ VI Rise and Fall  •  Gathering Storm
Official Spinoffs Sid Meier's Colonization • Sid Meier's Alpha Centauri (Alien Crossfire)  • Civilization Revolution • CivWorld • Civilization Revolution 2 • Sid Meier's Starships
Other games: Freeciv • Imperialism • Civilization: Call to Power • Call to Power II FreeCol • CivCity: Rome • C-evo • NewCol
Comparisons Comparison between Civilization games • Civilizations