I signed up a few weeks ago at http://blog.paperbackswap.com/ and just received my first book today!
Here's how it works: You list books that you wish to swap and search for books that you would like to read. You pay the postage when you ship books (usps media rate is very reasonable) and when you request books they pay the postage to send it to you. Other than shipping there is no cost involved.
You get credits for sending books and use the credits to request books. It's as simple as that!
When you first start if you enter 10 books that you are willing to swap you get 2 free credits to request books. With over a million choices I'm sure you can find something. I used my 2 credits on a novel and a cookbook (the cookbook should be arriving soon). Entering the books you wish to swap is SO easy.. you just enter in the ISBN number on the back of the book and it pulls up all the information.
I'm excited to read my new book, Conversations with the Fat Girl by Liza Palmer!