InstantMessengers with Jabber support:

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

Qt/KDE

• Psi, the canonical Qt client
• Kopete, KDE's main multiprotocol IM client.
• Konverse for KDE. Webpage was last updated in 2001.

Console

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

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

Java

• Greenthumb Java applet

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.
• The Coccinella, written in TCL/Tk. Jabber chat client with whiteboard communication - share a common window on your computer with two or more other Coccinella users. UserInterface sucks too, but has a great feature: shared whiteboard where you can do some cooperative drawing. Also, it implements mu-conference protocol's room administration options quite well.
• Nitro for MacOSX