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There are many other sites like Alexa, Yahoo and other. But google PR is the main factor. If you have google PR then no matter if your site doesn't have alexa rank. But you will surely have because many of them depend upon google.

PageRank is Google's way of deciding a page's importance. It matters because it is one of the factors that determines a page's ranking in the search results. It isn't the only factor that Google uses to rank pages, but it is an important one.
PageRank is a numeric value that represents how important a page is on the web. Google figures that when one page links to another page, it is effectively casting a vote ...