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 1,793,438 websites that are Alexa Certified Site Metrics Customers. We know of 79,383 live websites using Alexa Certified Site Metrics and an additional 1,714,055 sites that used Alexa Certified Site Metrics historically and 36,485 websites in the United States.
Get a list of 1,793,438 websites using Alexa Certified Site Metrics which includes location information, hosting data, contact details, 79,383 currently live websites and an additional 386,094 domains that redirect to sites in this list. 1,714,055 sites that used this technology previously and 36,485 websites in the United States currently using Alexa Certified Site Metrics.