Global Hunters

With this mod you can have the original L4D campains (and caracters) on L4D2. Its quite nice, because you can play the campains with the new weapons and new mechanics (and a lot more zombies):

http://nekoarcnetwork.com/l4donl4d2/index.html

To install, you need to first install "Left 4 Dead 2 Add on support" on Steam > Tools.

Afther that, just download the files on the page (the character pack is optional), unzip and put all the .vpk files on:
C:\Program Files\Steam\steamapps\common\left 4 dead 2\left4dead2\addons

With everything installed, just open a new game and select Add-on campain.