GOG's "DRM-Free Fall Insomnia Sale" - not sure when it ends.
101 games on sale, one at a time, in limited numbers. Some are free (these are going fast). Keep an eye on the front page in case a game you're interested in shows up - it's pretty clever marketing.
From the community post:
So what games will be on sale in our Insomnia promo? Bestselling classics and new games alike. There will also occasionally be some free games in super limited numbers (like, 20 or 30 copies)—if you're fast enough on the trigger finger to pick 'em up, that is. The deal will run from now until we're out of "stock" of games for the sale, and games may show up more than once.
More info here http://www.gog.com/forum/general/special_promo_fall_insomnia…
Assumed this deal was already posted, but turns out it wasn't!
EDIT: some rather motivated people on reddit have made a list of the games and prices involved: http://www.reddit.com/r/GameDeals/comments/1qjevg/gog_fall_i…
Great idea! Now I'll have two browser windows open all day…