SIM - Multiprotocol Instant Messenger

SIM (Simple Instant Messenger) is a plugins-based open- source instant
messenger that supports various protocols (ICQ, Jabber, AIM, MSN, YIM).
It uses the QT library and works on X11 with optional KDE-support.

SIM has a lot of features, many of them are listed at:
http://sim-icq.sourceforge.net/