I actually posted this in the other thread that was started about the same thing... so I am gonna cut and paste it here too !
Guys L4D is not finished.. granted everyone started getting bored with the same 4 campaigns however since they opened it up and users are creating their own maps.. there are some brilliant custom maps out there
Granted you have to download them and install them.. but its worth the effort. The vpk files are really easy.. you just double click on it and it installs it to the right directory.. you dont have to do anything but download & double click !
There are 3 good sites for these at the moment
L4Dmaps.com
L4Dmods.com
L4Dmapdb.com this one is pretty interesting cos it has 3 pages of campaigns that are either partly released or works in progress
I can recommend.. escape from toronto, death aboard, heaven can wait (if you want a real challenge) Silent hill.. a big download for an exceptional map.. its not quite finished yet and it takes a long time to play but so totally worth it.. also 7 days later
Some of these maps are already in final version and totally worth a look.
However, each of these sites have lots of maps for survival, versus and campaigns... I would recommend Helm`s deep and Buildead for Survival maps
It really opened up L4D and gave it a renewed interest for me !