I always go to
http://www.kindleboards.com/. Go to the Book Bazaar, Free Book Thread. It's updated each month. You will need to verify that the book is still free when you click on it, though. Sometimes books are only free for a short while. I missed the one day last year when the Lemony Snicket books were all free, but I've snagged a bunch of other great freebies!