Cache of others skins?

No, skins are cached in memory, however you can view other skins at: http://s3.amazonaws.com/MinecraftSkins/[USERNAME].png

E.g. My skin would be at http://s3.amazonaws.com/MinecraftSkins/mike4560.png

Minecraft does not cache any skins as files on disk, but for your interests for this topic I have some for you :-)

If you want to get a skin from a specific user from whom you know the username (with correct case!) then navigate to http://www.minecraft.net/skin/Username.png

For example: http://www.minecraft.net/skin/Notch.png to get Notch's skin.

I use this on our servers website to display pictures of online users and player cards showing the users appearance in game.

Skins are typically stored in your .minecraft folder. There is a skins folder, which contains all cached skins in multiple folders. These are .png files, however they are not named as such; you have to rename them. You also don't get to see the usernames, just a generated number.

On windows the folder is typically in %AppData%\.minecraft\assets\skins (type / copy this in the folder browser address bar).

Note: Please don't copy other's skins without permission; it is disrespectful.