I know this will probably cause dissent, but I think the time of Google (now Apache) Wave may have arrived :-) XMPP based (but hideously complicated) and can still do more than any other "chat" system out there that I've seen.
I immediately thought "oh, this looks like wave". The problem is that still don't think that this (or Apache Wave) is a whole quanta better than the good enough solutions that are out there.