http://www.jabber.org/
From the FAQ:
1. What is Jabber?
Jabber is best known as "the Linux of instant messaging" -- an open, secure, ad-free alternative to consumer IM services like AIM, ICQ, MSN, and Yahoo (see the IM quickstart). Under the hood, Jabber is a set of streaming XML protocols and technologies that enable any two entities on the Internet to exchange messages, presence, and other structured information in close to real time. Jabber technologies offer several key advantages:
2. What can I do with Jabber?
So far you can use Jabber to talk with other people who use Jabber and participate in Jabber chat rooms. You can also chat with users of some other IM services (see below). Depending on which Jabber client you use, you can also transfer files, receive headlines, and so on.
3. Can I use Jabber to talk with people who use other IM services?
You certainly can! Communication with legacy IM services is made possible by what we call "gateways". Jabber developers have created software for gateways to AOL Instant Messenger (AIM), ICQ, MSN Instant Messenger, and Yahoo! Messenger. However, not every Jabber server runs the gateway. See the Jabber User Guide for help setting up the gateways in your Jabber client.
Google Talk runs on a Jabber server
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