This is a cool idea: BookMooch.com. It's a points-based book trading system. You can list books you own and want to give away, then when someone requests one, you send it and earn a point. With each point, you can request someone else to send you a book. The only cost is the postage. If you sign up and list 10 books to give away, you get a starter point to get you going before you even have to send anything out. Should be good, assuming they get enough people and books in there to have a decent amount of interesting things available. I'm going to try it out.
Update: Within a day of signing up, I sent off one book, got credits for two more, and requested one to be sent to me. Good start!
P.S. You can view my wishlist if you want to send me something, or see the books I have available to trade in my inventory.