Kopete is a flexible and extendable multiple protocol instant messaging
system designed as a plugin-based system.  All protocols are plugins and
allow modular installation, configuration, and usage without the main
application knowing anything about the plugin being loaded.

The goal of Kopete is to provide users with a standard and easy to use
interface between all of their instant messaging systems, but at the same
time also providing developers with the ease of writing plugins to support
a new protocol.

Kopete is a big project which already supports messaging protocols like
Jabber, ICQ, AIM, MSN, Yahoo, IRC, Windows LANs, GaduGadu, IRC and SMS.