Head over to GOG right now for a free copy of Duke Nukem 3D, and celebrate the apocalypse with a nuclear blast or two. What more is there to say? The game came out in 1996, and it's still awesome.
In other GOG news, the site's holiday sale has put games ranging from the original Rollercoaster Tycoon to The Witcher 2 and Legend of Grimrock. Check it out for some delicious cheap games.