Psi is a leading instant messaging / real-time communications client for the Jabber network (using the XMPP protocol). It is cross platform, available for Windows, Mac OS X and X11 (Linux, BSD, Solaris etc.) platforms. It uses the Qt library.
It aims to be (and is famous for being) standards compliant and stable. It is also at the cutting edge of XMPP development, with several of the authors very actively involved in the XMPP standards extension process.