Bookmooch is a wonderful site that allows you to trade the books you have for the books you want. They have a wonderful selection, which changes all the time, based on what people add.
You add the book you want to give away, they give you .1 point. Each book you want to get costs 1 point, so if you list 10 books you get 1 book, list 20 and you get 2. There is a ratio you must maintain, but I don’t know what it is at the moment. When someone requests a book you have listed you get 1 point. You then mail it to them (usually for around $2.38 via media mail). When you receive a book, you let them know you got it and you get .2 points. So you ALWAYS get more books then you give!
I have gotten 37 books and given 30. For alot of romance novels or series of mass market paperbacks/childrens paperbacks people give you a 2 for 1 deal (mailing two of these books costs the same usually) by giving you a point back when you order two.
Basically its wonderful!
You can also check out swaptree.com and paperbackswap.com, but bookmooch is the best of them all!