We order most of our materials from Amazon, so I've been looking for a convenient way to tell if a book under consideration is in our catalog already.
Greasemonkey & a custom user script fit the bill nicely:
A few caveats:
Like most scripts, this one was designed to work with Firefox; I haven't explored getting it to work with other browsers.
It depends on the existence of ISBN for the item in question, so movies, older books, etc. would not work.
Others have added all sorts of bells & whistles: xISBN lookups to search for related titles, custom messages based on the status of items (on order, on hold, etc.), ... just search the UserScripts site for Amazon + library. For a later date!