The best example of explaining Kerberos from a practical viewpoint I've ever come across, is Bill Bryant's Designing an Authentication System:
a Dialogue in Four Scenes. Originally intended to provide an understanding of the conceptual ideas in Kerberos, as a way to promote implemenations of KerberosV4, it does a decent job, and is quite amusing if you ask me.
SET is not a de-facto standard, although intended and promoted as such during development. Even VISA, one of the sponsors and heavy proponents of SET, now promotes newer and more modern security standards for e-commerce.
The first practical case of X.509 certificates that comes to mind, is the widespread use of SSL/TLS to encrypt data transactions on websites over HTTPS