I've gotten a couple custom maps that we can play... There are a couple sites that you can find them on, but the two main sites that I find good stuff on are l4dmapdb.com and l4dmaps.com.
Here are some links to some campaigns that I have an incredibly hard time completing, even on normal, with BOTS. They're so intense, we have to have a team of 4 people.
Escape from Toronto: http://www.l4dmaps.com/details.php?file=78
Death Knell: http://www.l4dmaps.com/details.php?file=237
Fantazyland Castle: http://www.l4dmaps.com/details.php?file=77
I also recommend several others for us to play such as DEATH ABOARD, FARM OF DEATH, ZELDA, BACK TO SCHOOL, and COLLABORATION.
Here's an excellent SKILL map that requires you to make special hunter jumps to complete the obstacle course to get to the end. It'll really help hone your hunter skills as well as walljumping and other things i didn't even know the hunter could DO in this game. It's incredibly fun, but be warned, it's also incredibly frustrating: http://www.fpsbanana.com/maps/76114
All in all, if you guys want to just download all these and a few others (maybe 10 total, equaling about 300 Mb give or take) we can play some of these custom ones to escape the monotony of the regular maps that we're so used to.
Some of these maps (Death Aboard, Farm of Death, and Dead City (maybe more)) support VS matches, but I don't know of too many of our computers would be able to host a good match for 8 people in one of these maps (specifically DEAD CITY, as it lags even while doing regular campaign) Even my computer, which is pretty beefy, has a hard time hosting VS matches, and it's usually pretty laggy.
So gear up guys, download these maps, and let's get playing! Seriously, the newer content is fun as hell, it's new and interesting content to play around with, and there are more twists and turns, adding whole new levels of gameplay!
NOTE: I realize BACK TO SCHOOL's second map WILL NOT work in multiplayer. That's something that the creator needs to fix, but the first map is fun as hell in co-op on expert. We just have to wait for that guy to patch it.