SwapTree is an online exchange where you can swap books, music and dvds. It is incredibly easy to use and the overall idea is a great one. It is a form of recycling (since green is the new black) and you will find your slightly used possessions a good new home where they are loved (kind of like what Rudolph did with the island of misfit toys).
Basically you load your books and create a want list of your own. The website matchs your items with other peoples wants and vice versa. Basically all you do is pay to ship the book to the new owner. The interface is easy and the website provides a postage labelling system. It is a great way to help the environment, support literacy and screw off at work searching through thousands of free books.