(Disclosure: This site contains product affiliate links. We may receive a commission if you make a purchase after clicking on one of these links. Read our affiliate disclosure for more info.)
GameFly / GameFly UK has a daily deal up plus a Civ sale going on.
Must Have Monday Deal
Stick It to The Man! $7.49 (Steam)
Civ Sale
Civilization IV $5
Civilization IV: Colonization $5
Civilization IV: Beyond the Sword $2.49
Civilization IV: Warlords $1.25