http://www.paperbackswap.com

I know many of you have books sitting around your house that you don't read. This is a pretty cool way to get rid of them and get some books you might want to read (including children's books).
http://www.paperbackswap.com/.
Basically it is a program to swap your old books (not just paperback.) From what I understand when you sign up and list 10 books that you are willing to part with, you get 2 free book credits. You can choose from any books that other people have listed to swap and they ship them to you at their expense. Then when someone wants one of the books you listed you will get an email and you ship the book to them at your expense. When they receive the book, you get another credit to choose another book. They also do a referral program that you can earn free book credits for (details below). If you are interested, please click the link on the left and when you sign up and post 10 books I will get a book credit.

Oh yeah, there is currently no cost other than the shipping charge you pay when you ship books to people who have requested them. The site says shipping is usually around $2 per book.

Referral details:

Every time you refer someone to PaperBackSwap and they sign up, post 10 or more books and list your email address as the person who referred them, you'll receive an extra book credit from us as a way of saying thanks! We want to build the largest virtual paperback book club in the world. The best way to do that is through positive support from our members. The Referrals work as listed below.
1 - 10 Referrals = 1 credit for each
11 - 20 Referrals = 1 credit + $0.50 PBS Money for each
21 - 35 Referrals = 1 credit + $0.75 PBS Money for each
36 - 50 Referrals = 1 credit + $1.00 PBS Money for each
51+ Referrals = 2 credits for each