XCOM: Enemy Unknown is currently free to play on Steam through Sunday until 16:00 Eastern. The franchise is also on sale until Monday, October 7, until 10:00 Eastern. Players should also be aware that if they start a game through the free offer, they'll need to buy it before the weekend is over for $10 to retain their progress.
The free-to-play weekend is actually part of a publisher sale going on with 2K Games over at the digital distribution network. Amazon is also having a similar deal. We'll be watching your progress with interest, Commander.