As its name implies, Campus Book Rentals rents books. Right now they are offering a $29.99 special on any book for the fall semester. Search by ISBN, author, title or keyword. You can also click the link to enter to win a Textbook Scholarship (No purchase required)
A surprising number of academic textbooks are available as e-books, for e-readers including Kindle and Nook. eBooks in all academic disciplines
Amazon also has free shipping for selling books back, but pays with Amazon gift cards.