Kerberos is a client-server network authentication protocol which works on the basis of tickets to enable nodes communicating over a non-secure network to mutually prove their identity to one another in a secure manner.
There are a number of free implementations of Kerberos 5. These include:
Both Microsoft Windows and Sun’s Java come with implementations of Kerberos.
See RFC 4120 for full details of the Kerberos v5 Network Authentication Service.