Bookmooch. This might be the most promising online book-swapping idea I’ve seen.  Basically you sign up, let people know which books you’re looking to give away, mail the books and then get points for every book you’ve mailed.  You can use those points to then have someone mail you a book you want. Here’s how the point system is explained.

I haven’t used it yet but I’ll let you know what happens.  If it works this seems to get around the problem of figuring out who pays the freight for shipping items when you’re trading stuff online which has always seemed like a sticking point for me.  Looks pretty cool.