Openfire is a real time collaboration (RTC) server licensed under the Open Source GPL.
An ajax web jabber client
iJab is an Ajax-based web based instant messaging program for XMPP/Jabber. It is free software
Jabber/XMPP support in MoinMoin
Jake is a collaborative file sharing tool that allows you to securely and easily share files on your computer with other people.
Secure video calls, conferencing, chat, desktop sharing, file transfer, support for your favorite OS, and IM network. All this, and more, in Jitsi - the most complete and advanced open source communicator.
A publish/subscribe architecture is natural to other problem domains such as instant messaging