
Google Analytics is a free service offered by Google that generates detailed statistics about the visitors to a website. Its main highlight is that a webmaster can optimize his ad campaigns through Google Analytics' analysis of where the visitors came from, how long they stayed on the website, and their geographical position.

A great tool and a new layer to Google Trends. With Google Trends for Websites users can check and compare traffic data for different Web sites, including their competitors. Much more accurate than Alexa.