Voici la mise à jour annoncée la semaine dernière sur le blog officiel de L4D, avec entre autres la gestion des campagnes custom.
Détails dans la suite.

From Jason Ruymen

A required update for Left 4 Dead is now available. Please run hldsupdatetool to receive it. The specific changes include:

Client:
- Removed an infinite ammo exploit for molotovs

Add-on Campaign Support:
- Players download third party campaigns as single .VPK file
- Players can manage and select add-on maps and campaigns via game UI
- Added matchmaking support and download/update prompt for add-on campaigns
- Custom campaign support in Leader boards
- If not already registered, left4dead.exe registers .VPK extension to allow automatic install of add-ons
- Servers and the steam API have been extended to allow servers to send up a list of keys for matchmaking up to 1200 bytes in size

Left 4 Dead Authoring Tools:
- Fixed missing fgd error
- Added vpk.exe command line tool for extracting, creating, and appending .VPK files
- Removed info_versus (not used)
- Added info_gamemode which fires outputs for Coop, Versus or Survival depending on what game mode the map is loaded in
- Updated tutorial_standards example map, repackaged as ‘Dead Line’ campaign example add-on
- Sample content now includes source files for official maps

Jason