I love this idea (and thanks to Excrutiating Minutiae for suggesting it) - a book swap service called Frugal Reader. You list the books you don't want anymore (like the ones that still haven't sold on half.com, because there's a billion of the same title already for sale at 75 cents plus shipping & handling), and if someone else wants it, they request it. You send it via media mail to them ($1.42 for most paperbacks), and then you get a credit for a free book to request for yourself. When you sign up, if you list five books you get two free credits for yourself.
It's the online version of the paperback swap rack at my local library, except the selection is bigger and no one gets to take and never give. Obviously, Frugal Reader will work best if lots and lots of people sign up. I think most of the books available are pretty mainstream, but that is exactly the kind of books that libraries often don't like to buy. Please don't mock me for reading and enjoying Nora Roberts and Amanda Quick.