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 1,708,548 websites that are Alexa Certified Site Metrics Customers. We know of 61,813 live websites using Alexa Certified Site Metrics and an additional 1,646,735 sites that used Alexa Certified Site Metrics historically and 35,696 websites in the United States.
Get a list of 1,708,548 websites using Alexa Certified Site Metrics which includes location information, hosting data, contact details, 61,813 currently live websites and an additional 403,958 domains that redirect to sites in this list. 1,646,735 sites that used this technology previously and 35,696 websites in the United States currently using Alexa Certified Site Metrics.