Alexa
The company Alexa.com provides information about top websites, traffic stats, related links, online reviews, contact info, and more.
It is a California-based subsidiary company of Amazon.com, known for its toolbar and website. Once installed, the toolbar collects data on browsing behavior which is transmitted to the website where it is stored and analyzed and is the basis for the company's web traffic reporting.
There is some controversy over how representative Alexa's user base is of typical Internet behavior, but lacking other sources, it's data can still be useful to get an idea about a site's traffic.