[Air-L] Web traffic rankings and numbers

I'm wondering where other researchers go to find measures of particular websites' traffic as well as web traffic rankings for certain categories (i.e. news, etc)? Any sites that keep good historical records of this data? I'm also wondering if anyone knows of an article or other source that offers a good overview for thinking about the complexities of measuring and ranking web traffic and speaks to the pros and cons of different techniques? I'm writing about from a humanistic background with a desire to touch upon rankings data in a considered way. -- Anthony Nadler Assistant Professor of Media and Communication Studies Ursinus College