http://www.nielsennetpanel.com
2. comScore
http://www.comscore.com
3. NetValue
http://www.netvalue.com
4. StatMarket
http://www.statmarket.com
5. netScore
http://www.netscoreonline.com
6. InternetStats.com
http://www.internetstats.com
7. Netcraft
http://www.netcraft.co.uk