Like most things proprietary, OpenSource people have written their own InstantMessenger, called Jabber. Jabber is built on a robust model, similar to SMTP and HTTP, and completely unlike IRC's unscalable mess. It is based on a core router which speaks XML with small plugin modules handling all the various parts of the InstantMessaging.
Users are identified by way of their JID, which is similar in concept to ICQ's UIN, but looks like an email address, whateveryouchoose@your.jabber.server. The contact list is called the roster. Adding people to it is called subscribing to their presence. Subscriptions can be both ways (you can see them and they can see you) or only one way, but unlike most other IM clients, Jabber also lets you know when people have you on their roster: this appears as a subscription from them, not to them. People can be added to your roster either by their Jabber ID or by searching for them in the Jabber Users' Directory.
Jabber supports "Transports", server-side plugins which allow a Jabber user to talk to users of other InstantMessenger networks such as AIM, MSN, ICQ, YahooMessenger, SMTP, IRC, IMAP and many others. Thus, clients need not support any other protocol than Jabber itself.
There are many clients for MicrosoftWindows, Linux, various PDA's and other machines/devices. Here's a list of clients for Linux at the time of writing:
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GTK/GNOME :
** Gabber for GNOME (uses GTK)
** Gaim, a multiprotocol GTK2 InstantMessenger which has a Jabber plugin
** 
Gnome Jabber, a newcomer which is full of GTK2 goodness. (The web page says that the author has stopped development of this and is working with the Gossip authors.)
** Gabber2, the GTK2 port of Gabber. Still in development.
** Gossip, another GTK2 Jabber client
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Qt/KDE :
** Psi, the canonical Qt client
** Kopete, KDE's main multiprotocol IM client.
** Konverse for KDE. Webpage was last updated in 2001.
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Console :
** imcom for console (requires Python)
** jabmsg batch-mode Jabber message sender (requires Perl)
** CenterIcq, a multiprotocol Curses InstantMessenger which has a Jabber plugin
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MicrosoftWindows :
** Exodus for MicrosoftWindows
** Jabber.com's client for MicrosoftWindows. (Current version seems to only let you connect to Jabber.com's server)
** JAJC for MicrosoftWindows
** Rhymbox, MSN Messenger for Jabber. Looks really nice in Windows XP
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Others :
** Tkabber, written in TCL/Tk (requires a couple of extra TCL libraries) so this should run on any OperatingSystem that TCL/Tk runs on. It's UserInterface sucks, but it's very feature complete.
** Nitro for MacOSX
