Valve has revealed, via its Left 4 Dead blog, that it will update the PC version of Left 4 Dead with the ability to create and view user-made campaigns and maps this week.
The Steam update will officially release the Authoring Tools - which have been in beta testing for a few weeks already - and allow everybody to play community campaigns easily. All you need to do is download a single .vpk file and drop it into a folder on your machine, and then select it from an "add-ons" menu option in-game.
Add-on support has been added to the game's matchmaking system, and if you invite friends who don't have the custom map installed, they'll be automatically offered an option to download it.
There's no exact date on the update, but Valve expects to release it this week. More at the Left 4 Dead gamepage.