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 397,480 websites that are Alexa Certified Site Metrics Customers. We know of 62,791 live websites using Alexa Certified Site Metrics and an additional 334,689 sites that used Alexa Certified Site Metrics historically and 38,999 websites in the United States.
Get a list of 397,480 websites using Alexa Certified Site Metrics which includes location information, hosting data, contact details, 62,791 currently live websites and an additional 402,971 domains that redirect to sites in this list. 334,689 sites that used this technology previously and 38,999 websites in the United States currently using Alexa Certified Site Metrics.