ComScore is to the internet what Nielson is to Television. It is one of the largest Internet usage tracking companies in the world. A partial list of clients are

AOL
Best Buy
Borders
CareerBuilder.com
Clear Channel Communications
Columbia House
Digitas
Discover Financial Services
Eli Lilly and Company
Expedia
ESPN
Ford Motor Company
General Mills
Google
HP Home & Home Office Store
Hyatt Corporation
Interpublic Group
iVillage
Johnson and Johnson
Knight Ridder Digital
Mattel
Medscape (Web MD)
Mercado Libre
Microsoft
Monster Worldwide
NASDAQ
NAVTEQ
Nestlé USA
The Newspaper Association of America
New York Times Digital
Office Depot
OMD Digital
Orbitz
Pepsi
Procter and Gamble
Starcom IP
Terra Networks
Ticketmaster, LLC
T-Mobile
Tribune Interactive
Verizon
Viacom International
Washington Mutual
Yahoo!

If you really care to know more about them there is a wealth of information available about the company on the internet..