GCFScape
Overview
GCFScape is a small tool that enables you to browse through the GCF files that are utilized by Steam games for backups. This could be usefull for making model mods or extracting in game sounds.
Extraction
Extracting files from GCF's couldn't be simpler with this utility as it's like the Explorer folder tree format. But if you're still stuck, here's how to extract files:
When you install GCFScape make sure you associate GCF files with GCFScape so they'll open straight away, they'll look like this if they're associated:
Double clicking one of these will open GCFScape. Say, you opened the Counter-Strike Source Shared GCF, the program will open and appear like this:
Clicking the little 
button, you can browse the folders clicking on them so they expand and show subfolders like so, then clicking on a folder will show the contents to the right:
Right clicking a file will bring up a context menu from where you can extract from:
Clicking this will bring up a window where you can choose your save location:
Clicking OK will extract the file to your chosen location.