Bored playing the same old campaigns? Then check this site: http://www.l4dmaps.com/. Pretty easy to set up, jus download the map/rar/zip file and paste into l4d 1 or 2 directory for Maps. Jus check thru the map list on the site and post maps ur intereted in playing. To play together we all need the map u wanna play downloaded to each of our PCs so post the link if u find anything.

If you have no clue how to do it, heres a guide: http://www.l4dmaps.com/install-maps.php