BookMooch is a service that allows you to easily trade books with others. To use this book trading service, you enter in the book titles you have that you are willing to trade. Other people can then request these books from you, and when you send them to the other people you get points. With the points you accumulate, you can then request books from other users.

This book trading service is free but you do have to pay for the shipping of the books you send out. It's still a lot cheaper than buying them though.