So Paperbackswap ISN'T completely free but it is pretty neat and it's almost free. The basic idea is you list up all the books you own and and want to get rid of (any book, unlike the title suggests it doesn't have to be paperback), and then users can request books from you. You send the book (note: you DO pay shipping, but it's media mail so fairly cheap) and when they get the book, you get a "Credit" that you can use to get a book "FOR FREE" (minus what you paid in shipping of course).
It seems pretty neat, and they also have a feature where if you buy & print shipping off their site you get Instant Credit towards a book. The majority of people who use the site seem to be those into the cheap Harelquin romance novels, but there's all kinds of books.
They use a FIFO (first in first out) system to determine who gets a book once it's listed. Basically you put books on your wishlist, and you enter a queue for that book. When you're at the head of the line and someone posts the book, you get it! You can bypass this system by requesting a book directly from another user, and there's forums for book exchanges and stuff like that.
Check it out, let me know what you think!
EDIT: 50 followers! Wow, thanks you guys XD