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.
Valve are also apparently releasing the full L4D Authoring Tools, which will include the source files for all official Left 4 Dead Maps. You can read the whole thing on the Official Blog.