There are loads of different mods out there, new skins for the survivors, whole new campaigns and survival maps.
This one has to be my favorite mod ever, a few of you have already played it. It is a Helms Deep recreation from Lord of the Rings. as the game progresses you move further back through the castle, then at the end you run right the way though and say Hi to Gandalf.
-Helms Deep- Reborn
This campaign is very fun, although I haven't played it in a while
Dead Before Dawn Campaign
The mods just add a little bit of depth to the game, as the default maps can get quite repetitive.
To install the map files simply extract the .vpk files to here; ...\Steam\steamapps\common\left 4 dead 2\left4dead2\addons
If you don't have an 'addons' folder just create one and extract it in there.