SEO Quake is my favorite. You can Google that, or search for it in the add-ons section of FireFox. It allows you to show nofollow links, and check just about any statistic about your site and other sites that you would ever need. I have used it for a while now, and it has not let me down. The only thing is that right now the Google statistics are going to be off, they are going through some maintenance or something.