Ok, I don't know about plugins, but for those people who want this, there is a client-side mod for it. It is called chat bubbles, it might be updated for 1.6, to is created by the creater of Zan's minimap, which alone makes it awesome, and it itself is pretty awesome. Also, not only does it do these things but something I noticed when I had it, (before everything updated to 1.6 and mods got crazy annoying) is that in Massivecraft, when you send a private message to someone, what you said shows up over their head. But other then that, it does exactly what is suggested here.
I doubt there is or ever can be a plugin for this, but it doesn't matter when there is the mod that does the same thing with more work and irritation on the part of the users.
Yes, it can get crowded in the larger crowds in Regalia, but 1: it does not replace the chat box, and 2: it is still very well manageable. I would know, I managed it.
I hope this answers a lot of peoples questions.