XCOM: Enemy Unknown is available now on Steam for $40. The news represents a curious turnaround for the Mac version's developer and general Mac specialist Feral Interactive, who told us back in May it "doesn't release games on Steam."
A month later, Feral announced the arrival of the Mac version on Steam, which it says supports cross-platform multiplayer across Windows PC and Mac. Also, if you've bought the PC version on Steam, you'll have free access to the Mac version.
XCOM: Enemy Unknown launched a full-scale invasion of our Top 10 of 2012 list, eventually notching a commendable second place behind Journey. Since then, Firaxis' extraterrestrial strategy game has hovered across to iOS, where it's currently available for $20.