Zephyr is an instant messaging service that interacts heavily with kerberos to provide authenticated person-to-person and group messaging. By subscribing to the right message classes, you can be informed when someone logs in, when you receive mail, and even (does this still work?) be informed when your print job completes.
We run a zephyr server for CLUB.CC.CMU.EDU. It is currently cross-realmed with ANDREW.CMU.EDU and DEMENTIA.ORG.