Freeciv is a cross-platform, open-source project dedicated to creating a Civilization-like gaming experience.

You can run a binary of it on Mac OS X via X11.app — I’ve gotten it working relatively easily. The Freeciv wiki has Mac OS X installation instructions.

The game is just as addictive as the original and with the additional fine control you get in starting parameters with this project, even more worthy of replay. Graphics are definitely old-school, the controls occasionally leave something to be desired, and there is a definite lack of polish, but it says a lot about the superiority of the underlying game that all those problems are forgivable.

I haven’t ventured into networked multiplayer fun yet, but from all accounts it is spiffy.