It's not geared specifically towards college books, but it enables book-lovers everywhere to swap theirs with other members of the 2Swap community just by paying the postage to mail out a book from their collection that they no longer need.
That puts a credit into your account, which enables you to request a book from another member who will, in fact, pay the postage to mail you a book they have laying around and earn a credit themselves.
The intent is to get used books out of dusty old closets and back into circulation where they can be put to good use, creating less need for people to purchase new books and helping them to save a buck at the same time.
Of course the local library is often just down the street. But for getting old books back into circulation I'd have to say that this book-lending scheme gets an A+ from me.