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 – Education, Household Income
Other than demographics, you will also know about the various categories of topics that are covered by a particular website, what kind of ad formats are accepted on the site and which are the other related sites – this part is almost the same as Google Trends for Website but in Ad Planner, you can dig further into the web statistics of these other sites.
Google Ad Planner has been available to a select agencies until now but the good news is that anyone with a Google Account can now use this "very useful" tool. It doesn’t matter if you are an advertising agency or not.
If you display stats on your site, you may want to include the demographic data from Ad Planner as well since it will surely help potential advertisers make the right call. To learn more, check the Ad Planner Support site.