Alibris - Source for used and out of print books.

AddALL - The book search engine that allows you to comparison shop among 41 online bookstores including Amazon, Barnes and Noble, and Borders. You can expect the most complete book search and the best price Internet-wide when you use AddAll. They also search many used and out of print sources when you make an inquiry. This is pretty cool.

Abe Books - They advertise access to 20 million books. You ought to find what you need here including, if you have college students like myself, fantastic prices on many college texts.

Half-Price Books - Being a good Dallas girl (the home of Half-Price Books) I have to admit that these stores are my favorite stores in the world. You will truly never know what you will find. There are locations in sixteen states (though sadly not in Utah) so if you are fortunate enough to live near a store...make it a part of your Sunday afternoon routine. Otherwise visit their website. I've made incredible finds there.