We've already seen the Steam Autumn Sales, and the EA Origin sales, but if you were looking for something to buy from GOG.com, you're in luck - GOG.com has a massive sale which includes over 60 games.
A bunch of DRM-free classics, mixed with some newer titles are all on special. Some of my favorite games from yesteryear are on special, with Alone in the Dark 1, 2 and 3 on special for just $1.79. You can get a game like Chronicles of Riddick: Assault on Dark Athena for a dirt cheap $4.49, and Rollercoaster Tycoon: Deluxe for just $1.79.
A full list of games can be seen here, with some of the games available for both PC and Mac.