This page brings up a little Java-based realtime chat and private message system. You type your message into the box at the bottom of the applet, and everyone currently on the Chat page sees it. You can click the [Who] button to see who's on. Private messages are not implemented yet.
There are a few behind-the-scenes features that make this chat system unusual - e.g. it can be used through firewalls, and it doesn't require a persistent server - but as far as the user is concerned, it's pretty generic.