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 1,720,511 websites that are Alexa Certified Site Metrics Customers. We know of 102,320 live websites using Alexa Certified Site Metrics and an additional 1,618,191 sites that used Alexa Certified Site Metrics historically and 40,031 websites in the United States.
Get a list of 1,720,511 websites using Alexa Certified Site Metrics which includes location information, hosting data, contact details, 102,320 currently live websites and an additional 406,920 domains that redirect to sites in this list. 1,618,191 sites that used this technology previously and 40,031 websites in the United States currently using Alexa Certified Site Metrics.