This is an incredible website where members can swap books (and DVD's and CD's) with each other... for free*. All you have to do is sign up, add available books to your bookshelf and once you successfully send out a book to another member, you get a credit for a book of your choice! If you don't have any books to post, you can add money manually, but how many people have tons of books lying around that they don't know what to do with or are just collecting dust? This helps reduce your waste as the books you send and receive are being "recycled."
There are a few caveats to the website, as you have to print out the address label, mark the books sent and received, and add postage (but it's media mail, so most of the time the postage costs less than $2.50). You are also subject to only selecting books that other members have posted, so it's not the same selection as Amazon, but the more people who join and add books, means there will be more selection! It is such an incredible deal that I have been using the site for over a year now, and have been extremely happy with it.
www.paperbackswap.com
*There is the cost of postage to send books out, but using stamps feels like it's free!
