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
• 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

MacOSX

• Adium X, GPL, based on libgaim.
• Nitro
• Fire, has a crappy icon.

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
• 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 (requires a couple of extra TCL libraries) so this should run on any OperatingSystem that TclTk runs on. Its UserInterface sucks, but it's very feature complete.
• 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 mu-conference protocol's room administration options.
• Greenthumb -- a Java applet