I used to use paperback swap when I used to buy a lot of books. I hardly ever read books more then once, so I figured someone else could enjoy it. With paperback swap you get "point" basically. You send someone a book, when they get it you get a point. Then you can choose a book from someone else. Its actually really good.
It's an okay service.
Postage to send a book is about $2.00. I can find many of the books I am interested in from used book stores (local and online) in the $2-$4 range, so the savings isn't that great.
One other suggestion about the library, they may have an online component with ebooks/audio books that you can download from your computer. That is an option if you prefer not to physically go to your local library.