Amazon Free Books
Amazon offers a few ways of getting free kindle ebooks.
Amazon Prime Lending Library
If you have Amazon Prime (or Mom, or maybe Student) then the Amazon Kindle Lending Library will allow you to select one book a month for free.
Special Offer for Free pre-release Novel
In addition, under the special offers for at least the last two months, I have seen they make special offers where you may select one of a few pre-releases. The other day, I selected the one that sounded more interesting, and it turned out to be a Young Adult novel, and I got a free MP3 credit from Amazon!
Amazon Free Ebooks page