Online Kindness: Trade Books With BookMooch
I've been using BookMooch since May 2007 and I think that BookMooch is a fantastic online tool for trading books and getting rid of the books which you no longer want to hold on to.
How It Works
For the official explanation of the point system go to the BookMooch About page.
Giving Books
Sign up for BookMooch, then add some books to your inventory that you are willing to trade away, and you will get +0.1 points per book. If someone from your country asks for the book, then you can mail them the book to receive +1 point, and yes you pay the cost of postage, and if the cost is too large then you can decline the user's request. If the user is outside your country then you receive +3 points, and the cost of mailing is higher. When you're paying the postage to give away your books for free think of it as prepaying for books that you will receive later. Once the user that requested the book acknowledges receiving the book you will gain another +0.1 points.
Receiving Books
To request a book from someone within your country you only need 1 point, to request from outside your country you will need 2 points. Once you request a book the book's owner will either send you the book free of charge, or give you a reason why they cannot send you the book in which case you would not lose any points.

@erikvold