So we've been playing the shit out of the Left4Dead demo recently. It's a lot of fun and it's pretty much sold me on the full version; I'm getting a lot of enjoyment out of two of the twenty total levels as it is (out of four five-level campaigns), so my perceptions about its "shortness" don't seem to be a factor, especially as the game is nigh infinitely replayable. Throw in the sheer effort others have put into shoehorning zombie modes into non-genre games (UT2004, CS:S, etc.), I can't imagine the Steam community not leaping all over this.
... I had to pause here for twenty minutes as I wracked my brain trying to find it, but here's the best example: dotd_crookcounty. Look at this fucking thing. It's goddamned beautiful. Look at the sheer fucking size of this map. The download's somewhere along the lines of 80MB. (Yes, I realize it was originally a bomb defusal map, but the author repurposed it when he realized its true calling: zombies.)
Anyway, now that I've wasted a lot of time, I'm running two Left4Dead instances on the Xen server. They are both at xen.thisisnotatrueending.com; at the moment, the private server is on port 27016 (password is derp) and the public server is on 27017. To reach them, you need to enable developer console under keyboard settings, hit ~ to show the console, type openserverbrowser to bring up the server menu, then enter xen.thisisnotatrueending.com:2701# into the Add Favorite window, and then you'll be able to connect. You cannot add the server via Steam, it has to occur within L4D itself. (Hopefully that will change once the full game is out...)
I think that's about it, I had something else to note but forgot. Oh well!