Valve has announced details of an update to Left 4 Dead that is due out next week. To start, the SDK open beta is over, and the final version is being released. They're also adding a framework for easily sharing player-created content, accessible through the menu system in-game. In addition to that, they're revamping the matchmaking system to accommodate custom campaigns (like this Resident Evil 3 mod).
"Content authors will be able to package up their new maps, along with new posters, models, and textures, into a single .VPK file. To install and activate this content in-game, players need simply download the .VPK and double click on it. ... When creating or searching for a lobby, a new option named 'Add-on campaign' will let you select from among the add-on campaigns that you have installed. You can then find games or lobbies as well as create a lobby for that campaign. You can invite your friends, too. If they do not have the campaign installed, they will automatically be offered and an option to download it."