Valve Released "Dead Air" and "Blood Harvest" Campaigns for L4D2

Valve released the "Dead Air" and "Blood Harvest" Campaigns for free on PC and Mac, but they will be sold, for money, on Xbox 360 along with "Cold Stream" in the upcoming DLC. 

And here's the Cold Stream wallpaper:

If you want some backstory to the first L4D, check out this COMIC.

You can also buy L4D2 HERE for Xbox, and HERE for PC/Mac

They are all very cool campaigns, each with their own sets of easter eggs, which can easily be found on google or youtube. 

Have fun!