www.l4dmaps.com has a bunch of custom campaigns and versus maps to download, you have to host them locally, but a dedicated server on your pc should do it fine! enjoy

thought i'd pop this up since i just found it out, and have been looking for the solution for a while... when playing online over wireless, or doing anything online over wireless in vista and W7, you may experience lag spikes every now and then, this is due to the OS searching for new Wifi connections every 60 seconds or so... you can fix this by downloading vista anti lag (VAL) for vista, but there is no known fix yet for Windows 7... google vista anti lag for the website (free download)

Enable your console in the options menu in L4D, then go to your console by pressing the (~) key, then type "openserverbrowser" voila, you now have access to the regular valve browser search, you can see server latency, population, and whatnot, and in the custom tab, you can also search the "modded" variety of servers, definately worth a look, i've been playing a couple of servers which crack me up :) i'm talking low grav and adjustable difficulty on versus