Steam News announces a new version of Left 4 Dead will be automatically applied to the zombie shooter the next time you start Steam. As described below, the new version fixes an exploit, and offers improved support for modifications:

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