A new map has been released, Blood Harvest! Servers will go down immediately. Update details bellow.
Taken from Steam News:
Updates to Left 4 Dead 2 have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:
Death Toll campaign released
Fixed scoring in finale
Blood Harvest; The Bridge
Infected will spawn in front of the survivor team on their way to the Safe House.
Added Infected spawn locations on the hill by the Safe House.
Blood Harvest: The Train Station
Fixed exploit that allowed Infected team to trigger the onslaught.