People talk about "message notification". On a site like mine where we are encouraging people to idle in the chatroom, being notified of every message is not completely relavant. Instead what about a nudge system?
Next to each name on the "who is online" have a button or link that says "Nudge". When you press nudge it sends a popup that says "someone is trying to get your attention in the chatroom".
Next to your own name on the chatroom would be a button that can dissable nudges.
And people would have a nudge limit based on time. (Calculated from the nudge receiver, they would only be able to receive a nudge every 1 minute or longer or something, specified by the admin panel).
We are thinking about making this mod to phpfreechat for our website I think anyway. (Though some more discussion needs to go into it). We have a problem of people who are idling in the chatroom, missing a message aimed at them. Would you want us to include the code here?