As its name implies, Campus Book Rentals rents books. Right now they are offering a $29.99 special on books 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.
Get 20% off science, architecture, chemistry and education ebooks for the fall semester from ebooks.com
Or, rent a digital version of your textbook from Packback for $3-$5 a day, which sounds cheap until you add up the number of days in a semester.