I read a while ago (2-3 years) in a PC Gamer(?) magazine about a mod for I think Half Life 1 that put you against an unlimited amount of zombies. You had to build a fortress at the beginning out of many different components, such as force walls, and you could even build "machines" like a spinning razor thingy. After that, the swarm of zombies came at you and you had to last as long as possible. You could never win though. It was set in a giant open grass field. Anyone remeber what it was called? Its very similar to this video, but the graphics weren't as good and the gameplay was different. http://www.youtube.com/watch?v=3zZtw_5gd8M