The post-nuclear RPG that influenced just about every Western role-playing game today is now free—at least for the next 48 hours. Good Old Games, which publishes games online—mostly old, and some new—has made the game available for free as a way to garner the site a ton of exposure.
You can pick up a copy of the game for free right here after you create an account on GOG.
Be warned though, the server's being swamped with the announcement and over 50,000 people are trying to download the game at the moment, so you might have to wait awhile before you'll have a chance to pick up the best ever made. It's worth the wait.