A reader commented the other day that I had left his favorite textbook site out of my list of websites where you can buy and rent textbooks.
He was right; I had left it out. But his preferred site wasn't a place to buy or rent; it was a search engine. So I thought it would be a good idea to start a page of textbook search engines.
This list is by no means complete; I've never even tried to find all the possible sites. And the list is also mainly focused on paper textbooks, not digital textbooks. In fact, so far as I can tell only Inkmesh checks the prices of ebooks.
Textbook Prices by CampusBooks.com