Adium is properly the most used messenger on OSX or it’s at least the most popular one. It’s a Multi Messenger and supports a wide rage of protocols.
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 also available for MacOSX.
- Is-a-furry.org Quickstart [Link]
- Official Pidgin Documentation [Link]
- Pidgin plugins [Link]
- Mac Download [Link]
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.
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.