www.paperbackswap.com
Post 11 books and you get credits. Everyone gets a wishlist. You can search for books that you want and either order it or put it on your wishlist, at which point, the site will contact you when the book is available and it's down the list to your name.
The only fees are shipping (you pay the shipping on everything you mail out - but so does everyone else), and for some of the extra services - like if you want to generate mailing wrappers with delivery confirmation.
I've been a member since last June, and it keeps track of statistics such as how many books you've got posted, read, etc. It also keeps track of how much you've saved by using the site instead of buying new.
Finally, there's a huge community of users and forums on the site.
I highly recommend this site. Warning, though... It's addictive.