Whilst the site doesn't have the largest catalog of games, I always appreciate the effort GOG.com puts into making its sales interesting. Right now, there's an Insomnia sale happening which is taking a novel approach to flash deals. Each deal has a limited number of "copies" and once they're sold out, that's the deal over.
There's a counter on the main page of the site showing you how many copies are left and it's compelling viewing. I've just seen 200 copies of Fez and Baldur's Gate sell for a couple of dollars in less than a minute. It's a novel idea for a sale and I'm keeping a tab open all night to see if Privateer 2: The Darkening goes on sale.