There is a list of sites at http://blogsuccess.com/quality-guest-blogging-sites-the-pretty-big-list-for-2012
Guest blogging is a very good option to get backlinks to your website. But you must be a good article writer and creative thinking person in order to use this option better. Also, the list of blogs you get for guest blogging for free will not be worth. Instead you can contact some good blogs with your sample articles and convince them for a guest blogging chance.