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 364,038 websites that are Alexa Certified Site Metrics Customers. We know of 56,250 live websites using Alexa Certified Site Metrics and an additional 307,788 sites that used Alexa Certified Site Metrics historically and 29,331 websites in the United States.
Get a list of 364,038 websites using Alexa Certified Site Metrics which includes location information, hosting data, contact details, 56,250 currently live websites and an additional 106,151 domains that redirect to sites in this list. 307,788 sites that used this technology previously and 29,331 websites in the United States currently using Alexa Certified Site Metrics.