Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
Gajim - Full featured and easy to use Jabber client
Empathy - Apps/Empathy - GNOME Wiki!
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...
Dino - Dino is a modern open-source chat client for the desktop.