www.amazon.com (the most famous)
www.abebooks.com (shipping is often cheaper than Amazon)
www.isbn.nu
www.alldirect.com (this was my favorite, but they went out of business)
www.bestbookdeal.com
www.booksprice.com
www.campusbooks.com
www.bookkoob.com (UK?)
www.A1Books.com
www.aaabooksearch.com
www.bestwebbuys.com
www.bookfinder4u.com
www.bookfinder.com
www.addall.com
www.AnyBook4Less.com
www.firstandsecond.com (Indian; see Rupee-to-dollar currency calculator)
www.Bookpool.com
www.half.com
www.froogle.com ("google.com/shopping")
www.DealOz.com (was www.campusi.com)
www.bigwords.com
www.bamm.com (Boooks-A-Million)
www.barnesandnoble.com
www.alibris.com
www.textbooks.com
www.valorebooks.com
www.bookburro.com
www.virginmega.com
www.elephantbooks.com
www.1bookstreet.com
www.bookwire.com
www.totallycheap.com
www.bookfetch.info
www.bookburro.org (Firefox plugin)
New:
www.directtextbook.com
New:
www.fetchbook.info
New:
www.bublos.com
Other lists like this one:
Yahoo list of book price comparison sites
http://www.starburst100.com/top100books.htm
http://rootshell.be/~yong321/computer/buycomputerbook.html
http://www.softpanorama.org/Links/links2bookstores.shtml
The Public Library:
Wake Co., NC public library online catalog