Steam’s annual Autumn (or Fall, depending on where you are) sale has begun. There’ll be new deals each day until November 26 on the front page of the store, along with “flash” deals which will last for 8 hours each.
Currently the only Mac-related games on offer is the wonderful Limbo, and the just-discussed Walking Dead (there are also some Mac games in the “Indie Games” category). They’re also unhelpfully not showing which games are Mac-compatible on the sale page.
Be sure to keep an eye on Savygamer, which is tracking most of the deals as they happen.