Today, for you:
Postal 2 will be free at GOG until December 18th till 2PM UTC. All you need to do is log into your GOG account, find the Postal 2 giveaway banner on the main page (it's a little ways down) and click the green button. Done. Game will show up in your library shortly. Extras includes a manual, wallpapers, some kind of soundtrack, SMS tones, avatars, postalforms (whatever those may be) and a comic book.
Have a look if interested and enjoy.
That was a fun stupid game if ever there was one.