Seo How-To is a completely free website ranking checker tool which is very efficient for personal as well as professional usage. It is a simple yet extremely powerful tool checking the online ranking and backlinks (or inbound links) of different websites in the most important search engines like Google, and Bing. You will not only see the Google Pagerank, because SEO How-To also conveys information about the website traffic and social bookmarks, AND it serves as link popularity checker, which will tell you how popular your website link is. The age of a domain, the Alexa ranking, the DMOZ links and the Yahoo Directory links, and the Archived Pages(history) of a site are supplied too. Above all there is also a W3 tool that links to the html and xhtml check of websites and pages.
Please keep in mind that Google is momentarily experimenting with different techniques, algorithms and new "slave" servers. This makes that the results of Google itself are sometimes a little contradictory.