Feral Interactive released its OS X conversion of XCOM: Enemy Unknown in April, and since then, you've been able to buy it from the Mac App Store, Feral's Web site and other digital distribution systems - except for the ever-popular Steam service. That changed this week. Now it's available for Steam.
XCOM: Enemy Unknown is a real-time strategy game in which you control a secret paramilitary organization that protects the Earth from alien invasions. It's the presumptive successor to the XCOM series originally developed by Microprose in the 1990s.
Purchasing through Steam enables features like Steam Play, so you'll be able to download and play the Windows version on a PC if you wish.