Paperback Swap is an online bookstore that is a little different because it lets you swap books with other users on the site. If you have a book to offer, you list it on your profile. For each book you send, you get one credit. You can use that credit to request a book from someone else, or to get a discount for the books this store offers.
For registering on this site, you get two credits, which means two books free of charge.