A game which, as its name implies, is basically an open-source version of Sid Meier's Civilization franchise. Primarily based on Civ 2 in design, the goal is to amass resources, build cities, discover new technologies to build their units and allow for their civilizations to grow, and achieve victory. That can come by vanquishing all of the other civilizations through war, forming permanent alliances with all other civilizations, or launching a successful space program and reaching Alpha Centauri. As with the Civilization games, it can be very complex, with a rather intimidating number of technologies, units and resources to keep track of, but once you've gotten everything down, it's an addiction that's hard to beat. It does also support modding like the Civ games, so you can tweak everything exactly how you like it if you have the know-how.