GOG is having a fall sale, and it's a big 'un. The sale ends November 15th. Until then, 350+ games will be constantly on offer. There will also be 24 hour deals, with a new bunch being added every 12 hours. There's also a new daily bundle every 24 hours, some of which you can build yourself from a pre-picked selection.
One day distribution platforms will stop thinking of cunning ways to get money out of your wallet, but today is not that day. In build your own bundles, "The more games you select, the better deal you'll get", up to a maximum of five. On top of that, spending money on anything in the sale nets you free games at different tiers. Currently, spending $5 gets you System Shock 2, $15 Chronicles of Riddick and $30 Banished.
There are some great games on offer. Pillars of Eternity (50% off) Crypt of the Necrodancer (50% off) and Gone Home (75% off). There's loads more obviously, and those are just a few of my personal picks. Feel free to shout at me/politely highlight some of the offers that have caught your eye in the comments.