Psi is a capable Jabber client aimed at experienced users.  Its design
goals are simplicity and stability.  Psi is highly portable and runs on
Windows, Unix/X11, and Mac OS X.

Psi is minimal but powerful.  There are keybindings for just about
everything, Unicode is supported throughout, and contacts are cached
offline.  Security is also a major consideration, and Psi provides it for
both client-to-server (SSL) and client-to-client (GnuPG).