Kerberos is a hidden feature in Windows 2000, 2003 and XP. It is used for authentication in a Microsoft Network Environment from the client to Server and most people do not know that they are using it. The question was about the Internet. To what extent Kerberos is used ACROSS the internet is not published information from Microsoft or I am unaware of it usage.
Kerberos is one of many important security protocols used behind the scenes.
Here are links for those that wish a quick study of Kerberos.
MS Kerberos Summary