SEO toolbars are extensions on web browsers that can provide marketers and webmasters with easy access to SEO data about the webpages or websites they visit. Some toolbars can be downloaded for free while others require payment for premium features.
SEO toolbars can tell web users a number of things about a given webpage, including its keyword density, the number of pages indexed on that URL, and the number of backlinks that are currently connected to the page in question. SEO toolbars are useful to study your own website, or those of your competitors.