Alexa Tools
Alexa Rank is a measure of web site popularity. The Alexa Rank ranges from 1 to the number of sites in its database (0 = unranked). Owned by Amazon.com, the Alexa Rank utilizes the optional Alexa toolbar to track each and every web site visit. Each visit to a site counts as one visit (called 'reach'); multiple visits to the same site during a day only count as a single visit. Alexa also tracks the number of sub page visits to each web site (called 'page views'). The Alexa Rank is calculated daily (or so) by using values collected for 'reach' and 'page visits' and then factors in the values for the prior three months. Updates seem to occur every 3 days or so.
Bulk Alexa Rank Tool