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.
Miranda is a very slim and fast Multi Messenger. It’s a bit tough to configure, but if it runs, it runs. Miranda’s need on ressources depends on how much plugins you use. Miranda is also completely customizable and configurable. There are also many “flavours” of Miranda out there. Like “Miranda IM”, “Miranda Fusion” and “Miranda NG” – The differences are the preconfigured Plugins, the looks and functionalities.
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.
PSI is a very simple, lightweight Jabber client which supports many many featured provided by the Jabber/XMPP protocol. It’s available for Windows (also in 64 Bits, which is I think the only x64 Bits client on Windows) and MacOSX. It’s available in many languages.