Have you ever looked at your book collection the way you've stared at your closet of clothes - with an expression of despair and a cry of "But I've got nothing to reeeeaaddd!"? If that sounds familiar, then I've got a couple of websites that may well be the cheapest remedy for your problem.
Point your browser to BookMooch.com and PaperBackSwap.com, two sites that enable people to swap books they don't want for ones they do. It's a brilliantly simple premise. On PaperBackSwap, users receive credits - the currency of the site - for listing books. When a member requests one of your books, you pay the postage, print a mailing label and ship the book to that member. When you request a book, you pay only a credit. You receive one credit for each book you list, and PaperBackSwap gives you two free credits when you list your first 10 books.
On BookMooch, you receive 1/10th of a point for each book you list, and 1/10th of a point for leaving feedback about a book you received from another member. When you send a book to a member within the same country, you receive one point; you receive three if the member lives outside your country. It costs you one credit to request a book; two if requesting a book from another country. Also, you are required to send out at least one book for every two receive; sending a book internationally counts as sending three books.
My friend Matt gives PaperBackSwap a thumbs up - if any of you have used BookMooch, let the rest of us know what you think of it.