Dragon from GamersLastWill.com has posted an indepth description on how to play Left 4 Dead in 3D... that is, with 3D glasses.

You'll need a newish Nvidia card and a pair of 3D glasses. Using special Nvidia 3d Stereoscopic drivers, and giving the registry of a good old hack, it's possible to actually put yourself in the game.

So now you really will have something to be afraid of - especially when you're surrounded by a horde.