Google publishes a list of the 1000 most popular Web sites based on unique visitors to the top-level domain. The list is compiled by their (DoubleClick) Ad Planner group and shows estimates for the monthly number of unique visitors and pageviews. Not surprisingly, Facebook tops the list with 540M visitors and 570B page views per month.
Each site is categorized (e.g., as social network, web portal, search engine, etc) though some of these are surely wrong — e.g., #985, dropbox.com, is listed as “Myth & Folklore”. They say 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.”
If you want to play with the data, a Karl Seguin has downloaded the data, added some additional attributes, and made it available in json. That would make it easy to run your own analysis on them — category distribution, country distribution, average load time, etc.