The one that re-started it all!
GOG are giving away a DRM-free copy of Postal 2 for free until December, 18 2019, at 2 PM UTC. Simply scroll down their homepage until you see the "get for free" link.
Postal 2 is a sandbox first-person shooter that normally sells for $9.99. The GOG version is compatible with Windows (7, 8 and 10), and includes the first 2 expansions.
Bonus downloadable content includes a soundtrack, HD wallpapers, SMS tones, and a comic book.
This is a sweet offer considering I was planning on purchasing this title after backlog of games to pass is drastically reduced. That list is now a bit longer...
Image credit: GOG.com