SEOQuake is an add-on for Mozilla Firefox and Internet Explorer. It is a must-have Search Engine Optimization (SEO) and web site promotions tool for webmasters.
The tool can be used to retrieve and investigate SEO parameters of web pages on the fly. User can use SEOQuake to study other web sites SEO parameters so that his or her own web site can follow the best practise of other web site SEO configurations in order to achieve higher ranking in the search engine results.
When a web page is loaded in the web browser, SEOQuake will retrieve and display the results such as, Google PageRank, Google, Yahoo and MSN indexes, and Alexa rank, and etc. in the toolbar.
I like the built-in Keyword Density tool which will analyze and tell you the density of every word found in a web page being queried. Within the Keyword Density tool, you can also load Google keywords data that coresponds to the keywords it has analyzed.