IRC would probably be the simplest option as users don't have to register just enter their nick and join the channel.
Most data centres will not allow you to host your own IRC server, so you either need to use a channel on an existing network or rent your own shell to run it in.
An alternative is
Ignite Realtime: a real time collaboration community site which is a Jabber server, the client is also on the site. The downside to this is users have to create a user name and password which they then use to sign in to the service, however it may be possible to link its user authentication to an existing database.
You would need your own server or VPS to be able to host this, which I don't know if you have, but I've not come across a data centre that doesn't allow Jabber servers yet.