Book Burro
An excellent tool that was introduced to me lately is a firefox extention called Book Burro.
When Book Burro senses you are viewing a book on the web, it will open a small panel in your firefox browser. If you click on it, it will open a larger (but still small) window with choices in which to buy the book. It lists the online store name with the lowest price available. You can customize it to which online stores you like (such as Amazon Marketplace, etc.), and you can also choose local libraries (based on your zip) that might have the book available.
Extremely useful. If you are already running firefox, click here to install it.
