jabber.org. Jabber is an server/client based instant messaging system similar to ICQ that allows messages from server to client in its own native XML DTD. What's nifty about Jabber is that the server <-> client communication is only one protocol, and the communication between other services is done at the server level. Any future protocol need only be installed at the server level and the clients don't need to know a thing. Huzzah!
It's open source and cross platform (expect quicker porting to new platforms due to its open source nature but, more accurately, because of its easily replicated client structure). Its smart structure means possible porting to simple devices (think cellphone jabber). Or communications to booking systems.
I've got a jabber server running on our gateway and slowly coaxed the flatmates around from ICQ. One guy's running Acorn's RiscOS and don't expect that to have a client soon, but he just Telnets into the server and uses it there. Excellent.
(oh, and yes there are `client <--> client` instant messagers. They are as terrible as you imagine... it's WinPOPUP)