If you replaced anything in CS1.5 or earlier, you're probably wondering where all the default models are. With the introduction of Steam, all default models are included in a large cache file (counter-strike.gcf) in the SteamApps\ directory. Placing a model in the correct directory simply overrides this default model. This way, you never have to worry about backing up your default models before installing a custom model. If you don't like a custom skin after installing it, simply delete its files, and the default will be used once more.
GCFScape is a small tool that can open and extract from GCF files that are utilized by Steam for game content storage. It can be downloaded from nemesis.thewavelength.net
Below is all the default counter-strike files as of 10.01.09
Directory Listing of /counter-strike-default_files/