SEOquake is a free plugin for browsers that provides you with key SEO metrics for a specific page, along with other useful tools such as SEO Audit and many others.
Well all I have to say is I now have it covered what to use from now on for a free backlink checker and anything else that has to do with SEO. It even shows how many facebook likes you have. This is the best free SEO tool I've seen to date as well as probably the most useful browser extension ever. Let me know what you think when you check it out. Interesting in hearing feedback on users who may have given it a try. I'm going to sticky this thread because of how useful it is. It's available in the chrome store and also the firefox store.