FreeCiv is a turn-based single and multiplayer strategy game, and as the name implies, it's exactly what it advertises: a free version of Civilization. Albeit being a poor mans version, its lack of flair and detail does not cripple the quality. It's still enjoyable, and it's still very much like the original Civ. And guess what? It's available for the Mac as well.
Click here for the MacOSX version (15 MB), and here if you want the XP version (7 MB).