I read a short article in this month's Pages Magazine about an online system where members trade books among themselves. It's called Paperbackswap.com. There's no cost to join, but when you join you have to list 9 books that you're willing to swap. In return, you get three credits that you can use to acquire books that others have listed. After the first 9 books, other books that you swap are on a one-to-one basis.
The sender pays the cost of sending each book along using media mail (average cost is $1.59), and that's the only cost incurred. It's cheaper than buying a used book most places, and you get to clear books that you don't want from your own shelves. I haven't tried it (I usually just donate my books to the library, or send them to friends), but I think that it's a neat idea.