Yesterday I stumbled upon a fabulous website! It's called PaperbackSwap, and basically you "trade" books with other members.
I listed 10 books that I was willing to trade. By the end of the day, I had requests for three of them. I printed out labels (courtesy of the site), shoved them into padded envelopes (optional), and sent them on their merry way via the postal service. I get a free "credit" for every book I send, in addition to three credits just for signing up. So, I get to pick out 6 free books and have them sent to me free of charge! This is a book junkie's heaven.