Sid Meier's Civilization is the best strategy game of all times..

Civilization 3 is the latest release in the Civilization series. There is also a multiplayer expansion, called Civilization Play The World, which is buggy as hell.

FreeCiv is a free Civilization clone.

Civilization Call To Power is a rather bad Civilization clone. Apparently they got in trouble for not having the rights to use "Civilization" in the name of the game. Call To Power 2 was a big improvement on Civilization Call To Power. Notice the missing "Civilization" in the name.