I ran in to a site called Paperback swap a while back. It's a great site for free books - basically, here's how it works.
1) You post 10 books that you don't really read.
2) You get credit to request a book from someone else. No charge, not even postage.
2.5) If you use the link below or put in my school email address, I also get credit for referring you and I also get a free book :)
3) If someone requests a book from your list, you mail that to them at your expense (media mail is usually very inexpensive - $2 a book) - when the person gets that book, you get credit and can request another book from someone else.
Repeat as often as you like.
If you are looking for books and you're not a snob about the condition of the books, it's a great little site. I've received Infinite Jest, Moneyball, and other books from the site recently.