As Search Engine Land notes, Google is not only making this ranking available, it is also offering advertisers the option of purchasing ads exclusively on these top 1000 most visited sites.
Google AdPlanner compiled the data from "a variety of sources including anonymized, aggregated Google Toolbar data from users who have opted in to enhanced features, publisher opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research."
Interestingly, Google has excluded some sites from the list--notably, several Google properties as well as R-rated sites. The company provides this caveat: "Keep in mind that the list excludes adult sites, ad networks, domains that don't have publicly visible content or don't load properly, and certain Google sites."
Check out the top 13 most visited sites in the slideshow below. A third of the top 21 sites are Chinese, including Baidu, QQ, 163.com, and more. See the full ranking here.