Heimdal is a free implementation of Kerberos 5.

Kerberos is a system for authenticating users and services on a network.
It is built upon the assumption that the network is "unsafe".  Kerberos
is a trusted third-party service.  That means that there is a third
party (the Kerberos server) that is trusted by all the entities on the
network (users and services, usually called "principals").  All
principals share a secret password (or key) with the Kerberos server and
this enables principals to verify that the messages from the Kerberos
server are authentic.  Thus trusting the Kerberos server, users and
services can authenticate each other.

Homepage:
http://www.h5l.org/
