The Pagerank mini-bot discovers the Google Pagerank for a given url! With this tool you can also find the Pagerank of expired or deleted domains that are no longer active!!
Pagerank used to be one of the hottest SEO factors for ranking well in SERP's (search engine results). It was named after Larry Page, one of the founding members of Google. It was the first algorythm that the popular search engine used
to rank the results. According to Google: "PageRank works by counting the number and quality of links to a page to determine a rough estimate of how important the website is. The underlying assumption is that more important websites are likely to receive more links from other websites.
Nowdays pagerank value is greatly diminished but should not be completely overlooked.
More info at Wiki.