Google releases a list of the top 1000 most visited sites on the web, by unique visitors.
Google is not only making this ranking available, it's also offering advertisers the option of buying ads exclusively on these top 1000. Interestingly, Google has excluded some sites from the list--notably, adult sites, ad networks, domains that don't have publicly visible content or don't load properly, and certain Google sites. A third of the top 21 WORLDWIDE sites are Chinese, including Baidu, QQ, 163.com, and more.
Here's the list of TOP 100 IN THE US.