I recently (re-)discovered BookHQ.com, a book price comparison tool. Although it doesn't have a great search feature (it's best if you know the ISBN) and it doesn't account for such things as Amazon/Barnes and Noble/Powell's "free shipping" offers on many items, it is still a great resource.
Update: WorldCat lets you search for books online, at libraries, and offers a price comparison. It's not as useful as BookHQ for buying books, but for cross-referencing libraries it's priceless! (I found out about WorldCat via Open Library, created by Aaron Swartz (no relation), an interesting project but not terribly useful, since it doesn't draw upon the vast information already available online at such resources as, well, WorldCat.)