Good Old Games.com proposes for free Duke Nukem 3D for two days! You just need an account, and purchase it for free:
GOG.com proposed a while ago Shadow Warrior and Redneck Rampage with reduced prizes, you can also find games such as Blood or Kingpin there.
The installations included DOSBox and the game, plus possible add-ons and goodies such as soundtrack, manual, and other. They come as well with frontends for settings editing and multiplayer.
Come Get Some!