Here are sites that offer free books in exchange for honest book reviews.
Blogging for Books
If you decide to participate, you must follow each site's guidelines for reviews.
Example: BookCrash - read and review book within 2 months, review should be honest and thorough, post review on your blog and 1-2 other sites like Amazon, notify BookCrash that you've posted the reviews and include their links.
Once you've read and given your review, then you can be approved to get another free book. I've done one for BookCrash and am awaiting approval so I can get my 2nd book.
Know of any other free book sites? Have you tried any of these?