Paperbackswap.com is a new site where members can trade books for essentially free. It reminds me of the libraries at hostels where travelers can leave or take books as they please. You have to list 9 books to start off, and you get 3 credits. For each credit, you can request a book that some other member has listed, and they pay for the shipping. When someone requests one of your books, you just print out a sheet of paper that you wrap around the book, and you send it off - at your expense. Now you're scratching your head and saying, "wait, that's not free!" Well, my friend, it actually is, in that pay it forward kind of way because for every book you send, you get another credit which you can use to request another book. What a great idea, huh?
While I still like the public library, this is another great option, especially if you end up liking the book so much that you want to keep it, or you're one of those people who likes to take notes and highlight the book so extensively that it doubles as your bedtime reading and nightlight. And despite the name of the site, you can trade hardcover books too, exceptthey usually weigh a little more, so that may cost a little more to ship them.
For charter members, this is free for a year, but if it ends up being successful (which I could see it being), they may have to start charging $30-$40/yr. in membership dues. Get in on this while you can!