Those wanting to build their own take on the zombie apocalypse are being well looked after today, with Valve announcing an open beta for Left 4 Dead's software development kit.
If you jump to the "tools" tab in Steam, you'll see it available for download. Contained within are "modified versions of the same tools" used by Valve to make the proper game, so you'll be coming up with more than skins and sound effects.
Industrious fans will be able to build levels, create props, import props, edit lighting, etc etc etc. And if you're worried how this will affect your multiplayer sessions, don't, because Valve are updating the game's matchmaking system "in a few weeks" to accomodate all the new, user-created stuff.