Empathy - Apps/Empathy - GNOME Wiki!
HexChat - HexChat is a fork of XChat with bug fixes and new features.
Gajim - Full featured and easy to use Jabber client
irssi - Irssi is a terminal based IRC client for UNIX systems.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Kiwi IRC - A hand-crafted IRC client that you can enjoy. Designed to be used easily and freely.