I recently joined Paperback Swap. By posting books on a virtual bookshelf for other members to request, I am granted credits that I can use to request books. The first person in your household to post ten books is given two extra credits that can be used immediately to request books. Otherwise, credits are granted when you send a requested book to another member and they receive it. One credit equals one book, with audio books costing two credits. You pay postage when you send books, but none to receive books.
The selection is diverse. I have personally requested Driven to Distraction (a very useful book on ADD), Educating Esme: Diary of a Teacher's First Year, and The Nanny Diaries. All books came to me in perfect condition.
You can establish a wish list for books that aren't presently listed on the site, and receive e-mail notification when the book becomes available. You can also share your current To Be Read list, and join in book discussions.
If you don't have books to list, you can also buy credits for $3.65 apiece. Either way, it is a great deal, especially when used in conjunction with library visits and book sales.