"What is PaperBackSwap? We are a group of real people who have formed a Club to swap paperback books with each other. No gimmicks. No spam. No advertising. No kidding. We are not a large corporation trying to sell you something. We're just a group of real folks who wanted a way of trading paperbacks with each other through the U.S. mail. Please read the testimonials and you will understand that this book club is for real.
When another member requests one of your books, you mail it to them. Yes, you pay for the postage. But then another member returns the favor when you request a book from them and they mail it to you. And that way the books are always free because we are all trading books with club members!" [thanks to Laura Beth]
Link to Paperback Swap