Posting on the official Left 4 Dead blog, Valve's Yasser Malaika has revealed the Left 4 Dead team has been hard at work on a new update to bring custom campaigns to Left 4 Dead on PC.
According to Malaika: "Third party campaigns are designed to be installed using a new Add-On framework. 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. Alternatively, they will be able to drop it into a new "addons" folder. The list of installed add-ons will be viewable from a new item on the 'Extras' main menu flyout."
The update also features a re-tooled Left 4 Dead matchmaking system to provide extensive support for custom add-on campaigns, plus the official release of the authoring tools/SDK.