If you happen to be a Twitch Prime subscriber you can get Civilization IV: The Complete Edition for FREE until February 21st. More details available here.
Some nice short instructions from reddit on how to get this done:
“If you have Amazon Prime, and it’s tied to your Twitch account, there should be a Crown icon in the upper left near your username. Click it, and all available offers should be shown in the drop down menu. Just hit claim.
These games run through a Twitch game client.”