If you use the Firefox browser, go and download the SearchStatus extension. With this installed, the Google PageRank, Alexa rank is displayed for every site you visit on the web.
However, that's not all. It has a keyword density analyser, keyword/nofollow highlighting, backward/related links and more.
Many users have also reported that their Alexa rankings improved after it was installed on their browser. I too have it installed on my favourite browser.