Custom Map Time!
Left 4 Dead 1 Survival:
Dismember the Alamo - This is an amazing map and I wish a Left 4 Dead 2 version would be released. It's really easy to last a long time because the map is large and the horde spreads out instead of focusing entirely on the survivors, but it's still really fun to play.
Helm's Deep - Absolutely enormous map. Whoever made this has too much time on their hands. Fun to play around on, though. Doesn't feel like Left 4 Dead.
Left 4 Dead 2 Survival:
Resort to Violence - Hotel-type resort area. It's alright, but not amazing or anything. Does not feel like Left 4 Dead.
Le Village 2 - This map is beautiful, but it is poorly optimized and needs more weapons. Most of the space goes unused.
Back to Blood Harvest - Two new maps + Last Stand with L4D2 weapons.*
Tears From Heaven - Like Le Village 2, but a little bit better. Has a lot of stuff that seems randomly placed but makes for varied gameplay.
*One of the maps is a remake of the farmhouse finale wherein you wait for six minutes to unlock a gas can to pour into a generator to ride an elevator up to a platform. You can't get down from the platform once you go up, and you can use any gas can to activate the generator.