Google Doubleclick Ad Planner – “You can see a list of the largest 1000 sites worldwide, based on Unique Visitors (users), as measured by Ad Planner. This list is updated monthly as new Ad Planner datasets are released. The list defines sites as top-level domains. For each site on the list, you’ll be able to see: The site category; Unique Visitors (users); Page Views; Whether the site has ads.”