This hack puts a WorldCat button on Amazon book pages. Clicking on the button will show you libraries in your area that carry the book. I use Amazon wishlists to keep track of my books-to-read list, so this is very handy.
According to WorldCat, Google Toolbar's autolink feature is supposed to provide a similar service. However, it doesn't seem to work that well.
The practice of having third-party tools introducing links on web pages is controversial. Personally I think it's ridiculous -- end users (that's you) have every right to remix, modify, and augment content they download. But YMMV.
Apparently library-related mashups are all the rage.
Peninsula Library System Amazon.com Lookup
This script actually does the catalog lookup, so it will tell you if the book is in the system or not and whether it is checked out.
And it turns out that the Peninsula Library System has their own Firefox plugin now! But it eats up screen space, and doesn't do the automatic lookup, so I think the script linked above is superior.