This came in as a comment on one of the posts below:
Alexa Internet grew out of a vision of Web navigation that is intelligent and constantly improving with the participation of its users. Alexa has been installed based on millions of toolbars, one of the largest Web crawls and an infrastructure to process and serve massive amounts of data. For users of Alexa's Toolbar and web site this has resulted in products that have revolutionized Web navigation and intelligence. For developers this has resulted in a set of tools unprecedented in scope allowing whole new services to be created on the Alexa data and platform. Alexa does a lot more than simple rank web sites. They also have data services and webmaster services designed to improve the web. Alexa is a very powerful tool used to rank web site traffic. We can find out how our web site traffic stacks up against all our competitors. This is one of the most accurate freely available tools to find out how well your site ranks up against millions of other sites on the Web.