Pidgin (Gnome/GTK) [Link]
Pidgin is a Multi Messenger. This means you can use Pidgin for many protocolls. Not just Jabber/XMPP, but also ICQ, Yahoo, MSN, IRC, Facebook and so on. Pidgin is also very easy to use and runs smoothly on Windows. You are also able to extend Pidgin with many many plugins. Pidgin is available in all repositories of all bigger and more known distrubutions such as Ubuntu, Mint, SuSe, Fedora and so on.
Kopete (KDE/QT) [Link]
You can see Kopete as a QT twin of Pidgin (Some people will kill me after they read this :3). It’s also a Multi Messenger and a standart part of the KDE Desktop. (Personally, on Linux I prefer Kopete over Pidgin). Kopete is quite easy to use and looks very nice.
Gajim is a XMPP/Jabber only Client. It’s very stable (even it’s still in beta) and has a nice look and feel. Gajim provides a good documentation and is quite easy to use after a short time of getting used to the functions. Like Pidgin, Kopete and many other Clients, Gajim is also available in most Distribution repositories.