BookMooch

BookMooch is a book-swapping community. It's more like a little economy, really. There are some flaws in the system, but essentially it's a great idea. You list all the books in your collection that you no longer want, and can then request books from the lists of other members. I received my first 'mooched' book today; Daniel J. Boorstin's The Discoverers: A History of Man's Search to Know His World and Himself.