Today Google announced Ad Planner, which is a free tool advertisers can use to track the popularity of websites. Other services like Hitwise and Nielsen are $subscription$ based. The whole idea behind this is to help agencies decide where to place ads and it works by simply entering information on a desired target audience and Ad Planner will display the most visited sites based on the criteria. Currently, it is in testing mode and users must apply here. Google just keeps on rolling out the products.