Empathy - Apps/Empathy - GNOME Wiki!
Gajim - Full featured and easy to use Jabber client
CoyIM - CoyIM is a safe and secure chat client.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Miranda NG - Miranda NG is a successor of a popular multi-protocol instant messaging client for Windows - Miranda IM. Very light on system resources and extremely fast.
Dino - Dino is a modern open-source chat client for the desktop.