Free children's (and grown-up) books
Here's how it works - you clean out your bookshelves and post books you don't want on paperbackswap.com. When someone selects your book, you mail it to them. In exchange, you get to pick books that other users have posted. So the only thing you pay is postage.
They have a large selection of children's books. The only issue is you don't know what kind of condition the book is in until you get it in the mail. It's kind of an honor system. I've only been disappointed once.
I've been using PBS for a few years now. I've saved well over $500 by swapping books rather than buying them at the store. I love this service. Its easy, fun and satisfying to share a good book with another reader.
Swaptree.com is another media-swapping service that also allows you to swap CDs, DVDs, and video games. Has full item condition rating and user reputation system.
Fantastic. Thank you so much for sharing the site. ;)