SEOquake is a nice browser toolbar that shows you a wide range of important SEO metrics across SERPs of Google, Yahoo, Bing, Yandex, and Baidu: domain age, PR, web archive, Alexa rank, number of backlinks, traffic, and many others. It also appears in the form of a stripe with the same metric set over websites when you visit them and as a more informative dasboard with the site info when you click on the SEOquake icon that appears in the browser after installation.

Additionally, the plugin provides 6 website reports: Page Info, Diagnosis (Page SEO Audit), Internal Links, External Links, Density (Keyword Density), and Compare URLs/Domains. The tool is quick, reliable, and highly customizable to the specific user requirements.