If you haven't play Freeciv, it's a good opportunity to try it. It's a freesoftware which is based on Civilization™. I've been playing Freeciv recently.
It's very easy to install Freeciv on Gentoo Linux: just emerge it. It's not very easy to install it on Mac OS X Snow Leopard.
There are 3 ways to install Freeciv on Mac OS X Snow Leopard. (1) 2.1.9 binary package for Snow Leopard, (2) 2.3.1 with sdl on Homebrew's Homebrew-games, or (3) jinhao's patched version of Homebrew formula. Dont choose (1) or (2).
Freeciv has 2 ways to provide GUI interface; sdl and gtk2. sdl interface is very inconvenient. gtk2 interface is much much better but it needs lots of compiling. Still I recommend gtk2 version for everybody.
Note that don't start Freeciv server on Mac OS X. I don't know why but it consumes all the CPU power. Singleplay in Freeciv simply means it starts server and connects itself. Start a server on a Gentoo machine and connect there to play.