Bookfinder - Used to be called the MX Bookfinder. This one used to be everyone's favorite, but recent changes have not been welcomed by everyone. It can be a bit complicated to use.
Books & Book Collectors - Steve Trussel has set up a very useful site with the ability to search many databases on the web. It's similar to the MX Bookfinder, but you search each site one at a time instead of all at once as with the MX Bookfinder. While this may seem time-consuming, you can actually search many sites that the MX Bookfinder does not.
AddALL - Has separate pages for new and used books. At the moment, this site appears to be a little quicker than Bookfinder, and is the one i use most of the time.
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