I don't use any at the moment. However, at one time I was using the Moz toolbar, and I think the Google PR toolbar. I can't really remember. Who has used those? Well, Google PR is obsolete these days. Either it's a secret or diminished.
Currently, I do not use any browser add-ons for SEO, I have used a couple of add ons on my Firefox browser in the past and I gave up because the add ons slowed my browsing experience. If you want, you can try keywords everywhere., It is a good browser add-ons.