InstantMessengers with Jabber support:

GTK/GNOME

• Gaim, a multiprotocol GTK2 InstantMessenger which has a Jabber plugin
• Gossip, another GTK2 Jabber client.
• gnome-jabber - but development of this has halted, and merged efforts with the gossip team.
• Gabber for GNOME (uses GTK)
• Gabber2, the GTK2 port of Gabber. Their website says that development has paused unless/until someone steps forward to lead the project (Oct 2004).

Qt/KDE

• Psi, the canonical Qt client (cross-platform - runs on Linux, MacOSX and MicrosoftWindows)
• Kopete, KDE's main multiprotocol IM client.
• Konverse for KDE. Webpage was last updated in 2001.

Console

• CenterIcq, a multiprotocol Curses InstantMessenger which has a Jabber plugin
• imcom, requires Python
• jabmsg batch-mode Jabber message sender, requires Perl

MacOSX

• iChat AV, requires OS X 10.4 Tiger.
• Adium X, based on libgaim. GPLed.
• Nitro
• Fire, has a crappy icon.

MicrosoftWindows

• Exodus
• Jabber.com's client. Current version seems to only let you connect to Jabber.com's server
• JAJC
• Pandion, MSN Messenger for Jabber. Looks really nice in Windows XP. Previously known as RhymBox.
• Neos MT Win32 Jabber client that supports digital whiteboard, audio & video over IP, and file transfer with history.

Others

• Tkabber, written in TclTk plus extra TCL libs. Should run on any OperatingSystem that TclTk runs on and is very feature complete, but UserInterface sucks.
• The Coccinella, also written in TclTk, and also has a sucky UserInterface. However, it boasts a shared whiteboard where you can do some cooperative drawing with other Coccinella users, and good support for the multiuser room administration options.
• Greenthumb — a Java applet