I have personally never rented a textbook, prefering instead to borrow mine from the college library or get them cheap through http://www.isbndb.com/, but I recently came across a Yahoo article on strange things to rent.
Chegg is a textbook lender that charges just 10 to 30% of the purchase price of a textbook to borrow it for a quarter or semester. Though you have to pay to have the book shipped to you ($3.99 Standard is the cheapest option), you can return it for free through UPS after the borrowing period ends. To sweeten the deal, Chegg plants a tree for every book you rent.
They offer a 30-day "Any Reason" guarantee, which begins the day the book is shipped to you. This comes in particularly handy if you happen to drop a course, or discover that you don't really "need" the textbook. How many students have discovered that their professor only alludes to a particular book in passing, after having spent $50 on the book? Annoying, to say the least. This sidesteps the issue nicely.
They calculate that "A student, on average, will save over $500 a year by renting compared to buying textbooks." However, "the savings are calculated by subtracting each book's rental fee from the book's publisher suggested list price." Maybe I'm the exception rather than the rule, but I never pay the publisher's listed price. Talk about inflation! However, 10-30% of the list price is still a really good deal.
I'm definitely going to give this site a try for my next online class. Paired with isbndb, this could mean some real textbook savings!