the yahoo! buzz index counts the total number of people searching for specific subjects every day on yahoo.com. the data is collected from the yahoo! search log files.
buzz leaders are the subjects with the greatest buzz score on a given day. these subjects are the most searched subjects on yahoo! for that day. the buzz index weekly report recaps the top 20 searches for a week and gives a good summary, what created the most “buzz” in that week in america.
alexa also lists the top 100 visited sites based on the usage patterns of alexa toolbar users over a rolling 3 month period. in general alexa gives you more information about the sites you visit.