Better World Books is an online bookstore that offers free worldwide shipping. They offer new titles as well as used books and textbooks. They are affiliated with over 2300 college campuses and 3000 libraries. So far, they sold over 117 million books for $15 million, and put that into funding for literacy and education.
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.