PageRank

While the name is PageRank, the add-on can fetch tons of other SEO related information for the current web page including the Alexa Rank, Whois details, geo location of the hosting servers, Google Cache status and more.

You can use the dashboard to find the relative traffic of a web page and there’s also links to Archive.org should you be interested in looking at the older versions of a website.

You’ll also find easy links to important webmaster related sites like the W3C link checker, Open Site Explorer, Google Webmaster Tools and few more.