A few people have asked me how to install custom maps for L4D, so I wrote this little guide to help you out.
Step 1: Download GCFscape:
http://nemesis.thewavelength.net/index.php?p=26
Step 2: Open the "counter-strike source shared.gcf"
Step 3: Locate and extract the "models" and "materials" folders to your desktop.
Step 4: Open "source materials.gcf"
Step 5: Extract "materials" folder to desktop.
Step 6: Open "source models.gcf"
Step 7: Extract "models" folder to desktop.
Step 8: Copy both the "models" and "materials" folders to C:\Program Files\Steam\steamapps\common\left 4 dead demo\left4dead
All CS:S maps should function with L4D now. You can play a bunch of NIPPER's maps on l4d6.joe.to
Unless you're a total badass like us, NIPPER was probably right when he said