Bookmooch has been one of my favorite sites for awhile.
Here’s a bit of info from the about page:
BookMooch is a community for exchanging used books.
BookMooch lets you give away books you no longer need in exchange for books you really want.
Give & receive: Every time you give someone a book, you earn a point and can get any book you want from anyone else at BookMooch. Once you’ve
read a book, you can keep it forever or put it back into BookMooch for someone else, as you wish.
No cost: there is no cost to join or use this web site: your only cost is mailing your books to others.
Points for entering books: you receive a tenth-of-a-point for every book you type into our system, and one point each time you give a book
away. In order to keep receiving books, you need to give away at least one book for every two you receive.
Help charities: you can also give your points to charities we work with, such as children’s hospitals (so a sick kid can get a free book
delivered to their bed), Library fund, African literacy, or to us to thank us for running this web site