online chat.

I think facepunch or garrysmod.org /com should have like online chat.
Where who ever wants to talk can just go into and start directly typing.

If you have Steam (which as far as I know pretty much 75% or more Garrysmodders do, as this is how they got the game altogether) you can already chat with your friends as your browse.

That would require more work from the mods or new mods entirely.

…or a php/flash/java/javascript(xml?) script chatbox.

Well, I can imagine it now.

IRC…?

facepalm