If you haven’t played Left 4 Dead, let me sum it up for you: AAAARRRGGHHHHHHHHHH!
Maximum PC has a great how-to on making Left 4 Dead maps using a combination of Valve’s Hammer World Editor and Google SketchUp. It’s a very detailed tutorial, going through 11 whopping (and add filled) pages of images and instructions. According to one of comments the maps you make will work with the Left 4 Dead 2 SDK when it comes out.
If modeling the real world has left you wanting or you stomped your way through Left 4 Dead and you need a new challenge, grab your SketchUp skills and start building your own personal zombie apocalypse.