Gajim
Gajim is an instant messaging client for the XMPP protocol which uses the GTK+ toolkit. The name Gajim is a recursive acronym for Gajim's a jabber instant messenger. Gajim runs on Linux, BSD and Microsoft Windows. Released under the GNU General Public License, Gajim is free software. A 2009 round-up of similar software on Tom's Hardware found version 0.12.1 "the lightest and fastest jabber IM client".
Philosophy and features
The goal of the Gajim project is to provide a full-featured and easy to use XMPP client for GTK+ users. Gajim uses PyGTK as GUI library, so it does not require full GNOME to run, though it works nicely under a GNOME environment. Some of its features:
Tabbed chat windows
Group chat support (with MUC protocol)
Emoticons, Avatars, File transfer, URL grabber, Bookmarks
Systray icon, Speller
TLS, OpenPGP and end-to-end encryption support (OpenPGP not available under Windows until version 0.15), including SSL legacy support
Transport Registration support
Service Discovery including Nodes