Paperback Swap is the coolest of the cool. I’ve only been there for a month and I’ve already rebuilt my keeper shelf and gotten tons of children’s books for the Luke. It’s a fairly simple system. When you sign up you enter all the books you want to get rid of. You can enter them by their ISBN, so it’s really easy. When someone requests that book you mail it to them and receive a credit, which you can use to request another book. One book costs one credit. They also give you two free credits for signing up. They say that at some point they’ll probably start charging an annual fee for the service, but as of right now it’s totally free.
You can print out a wrapper from the site that already has the amount of postage due on it. That way you just stick the stamps on and put it in your mailbox. Couldn’t be easier.