I use Bookmooch -- very similar service. I have gotten tons of books and have sent out an equivalent number for $2-3 each. Not completely free because of the media mail postage, but a bargain. Another nice feature -- you can easily upload your Amazon wishlist into it, and they'll automatically notify you when/if books your list are offered by other moochers!

No affiliation, just a happy member: http://www.bookmooch.com