Valve has recently released a beta version of a community map which has been planned for release on the Xbox 360 as well as PC/Mac in the next update. The map Cold Stream is an outdoor adventure pitting you against zombies and special infected with a few twists to add to the fun... or against it depending on your style.
The main one being the use of current in the stream meaning you can be whisked off from your team if you're not careful. There's misty tunnels, daring paths across fallen down trees and a desperate climb to the escape helicopter.
Whilst you can go and play the beta, for free, right now if you own a copy of Left 4 Dead 2 on PC/Mac, we have put some screenshots up in the gallery and also a video playthrough below of the first 2 chapters of the total four.