Harper Collins' "First Look" program lets readers write a short essay on why they'd like to review a variety of books (including mysteries) and several winners receive a copy prior to the publication date. If you "win," all you have to do is send a short (100 words) review which they may use on their website.
St. Martin's Press' "Read-it-First" program sends excerpts of new releases right to your e-mailbox. You get a short exerpt each day as well as a newsletter and chances to win free books.