I asked the question a few days ago: "Where can I go to get pricing from multiple sources for books by ISBN?" My queried friends did not know, nor did I.
Amanda pulled up BookSense, but it is dedicated to just searching off-line booksellers and requires a fair amount of information about your location to get started. I wanted something that I could use as my standard quoting for the web site, so I needed something a bit more generic.
Today, I happened across Bublos.com. This site provides a list of search results from a large number of online resellers (but, sadly, no offline booksellers that have no online component). It is likely that I will start using it for my standard book references, since it allows you to jump to any of the prices directly and without further ado.