The Pinterest bookmarklet in a Google extension format
I didn't like the icon on the other extensions so I made my own and used http://sandbox.self.li/bookmarklet-to-extension/ and the Pinterest bookmarklet to cobble together the button.
** update 12/12/12 **
Hold the phone! Pinterest have finally released an official extension. Which appears to do exactly the same as this one. Exactly. The. Same. Consider this extension retired.