Alexa's certified program and pro metrics.
http://www.alexa.com/siteowners/certify
Analytics and Tracking · Audience Measurement · Visitor Count Tracking
Get access to data on 1,697,464 websites that are Alexa Certified Site Metrics Customers. We know of 72,902 live websites using Alexa Certified Site Metrics and an additional 1,624,562 sites that used Alexa Certified Site Metrics historically and 38,254 websites in the United States.
Get a list of 1,697,464 websites using Alexa Certified Site Metrics which includes location information, hosting data, contact details, 72,902 currently live websites and an additional 407,052 domains that redirect to sites in this list. 1,624,562 sites that used this technology previously and 38,254 websites in the United States currently using Alexa Certified Site Metrics.