Freeciv

Freeciv is an open-source, free-to-play, single-player or multi-player, computer game developed independently (since 1996), inspired by Civilization: it "was created as a multiplayer version of Civilization™". Its current main downloadable versions include one that is very like Civilization II but with distinct improvements. Active development continues on several fronts. Freeciv multiplayer is enabled by having one player host a server.