I mentioned in an earlier post that I was once addicted to games like Sid Meier's Civilization.

Sid Meier's Civilization IV (Amazon affiliate link)

Open Source supporters may find it interesting to know that there is a clone of it that runs on Linux, Mac OS X, and Windows.

Freeciv 2.0.8 (released March 6, 2006)

I tried it years ago and although the interface at that time (I tried it on a Linux box) was a bit different from the original version, I found it quite playable and entertaining. Now, if only there was a port for a portable device like a Pocket PC, Smartphone, or Sony PSP :-)