SEO quake is the most popular and it's the only one that I have used but there are others you could give a try. You just have to search the Google chrome or firefox extension store to see what suits your take.
I have used SEO Quake and a lot of mother SEO tool bars as well as SEO extensions. While these toolbars and extensions will help you understand certain metrics, these tools and extensions will also make your browser slow.
Years ago I used to use so much for SEO but nowadays I don't tend to as much. I honestly didn't even think to look for an extension for SEO but now that I know, I am going to take a look.
One SEO tool that I find very useful these days is called Keywords everywhere. I use it for chrome. This tool will give you information on keywords thus making you easy to implement keywords on your content.
