Recycle, reuse, reread! BookMooch is totally a website of my dreams. While I've been marveling about finding the perfect Netflix-for-books service, I never even thought about the green option! BookMooch is light years ahead of me — they provide the means for you to to trade and receive used books — the only cost you incur is the price to get your pages in the mail.
When you type in the books you're done with and want to give away, you get a tenth of a point and you get one point when you give a book away on BookMooch. Each point is redeemable for one book that's been offered on the site by another user. For every three books you receive, you have to give one away, just to keep the system going. If a book you want isn't in the database yet, put it on your wishlist and you'll be notified by email when it comes in! This gives mooching a really, really good name.