BookFinder.com searches the major online booksellers including Amazon, Ebay, Alibris, Abebooks, and others. You can search by author, title, or ISBN, and it shows results for both new and used books sorted by price, shipping included. I’ve found it extremely useful, and have even found good deals on textbooks there. One drawback is that it probably can’t take into account discounts such as that free shipping deal from Thriftbooks (which I didn’t know about—thanks, Betty!).