Here is a quick plug for Paperbackswap.com
Simply put, you list books that you no longer plan on reading. After listing 10 books you receive 2 credits, which you can use to request books from other users. If you request a book, you get it... for free. If someone requests one of your books, you pay the shipping costs to send it to them, but also receive an additional book credit.
Brittany and I recently started using PBS to get books that we have on our Want To Read lists. So far it has been pretty effective for us, as we've received three books. The only cost was sending our books to other users Shipping ended up being about $2.75 a book, which is a price pretty hard to beat even with shopping used bookstores or half.com. The only catch is that you're relying on someone out there having the book, which isn't always the case.
If you want to try it out, here's a link: Swap Books for Free - PaperBackSwap.com
Note: I'm going to be linking all of my Book posts to Paperbackswap.com until further notice. I previously was using Amazon.com's affiliate links, but that service has been ended for Illinois residents.