Google Ad Planner is a pretty useful tool for advertising agencies to help them know more about websites you plug in the site name and Ad Planner will show you loads of useful data about that site. For instance, you get to know about the monthly page views (per country), unique visitors, impressions per day, average time spent by visitors on the site and visitor demographics (like age, gender, income, education, etc.). Web stats comparison of some tech websites Visitor demographics -   read more »