FreeCol is an open source clone of the Colonization game. I admit, during college I was a bit addicted to this game, my style of strategy gaming.
For those who are not familiar, this is a strategy game where your objective is to Colonize the entire world with your nationality. So, you build your cities, grow them, build an army and expand your borders. Sounds pretty simple but for people like me, fun stuff to do for hours and hours.
FreeCol isn’t yet on its full release version yet, it is still on 0.7.4 about to release v.0.8. But, from the looks it is working pretty well. It runs on Windows, Linux, and Mac OS X.