Another retro-fest by GOG in the vein of their Fall Insomnia Promo last year (which was apparently quite epic with entire communities staying awake for the sale).
The link page features a limited number of copies of a heavily discounted game (between 50-90% off); once they run down to zero another 100-300 copies of a different game come online, until all 100 games have had their turn. No telling what could be up there, so far I've seen Sin: Gold, Hammerwatch and Gobliiins pack.
Some free games will be included in the promotion.
Managed to grab Sin: Gold for 60% off at $3.99 (cheapest it's ever been and boy did they go fast).
I was just about to post this bargain but you beat me to it. Kudos to you.