Empathy - Apps/Empathy - GNOME Wiki!
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
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Pinger - Pinger lets you text and talk free with the people you care about most, anywhere on the planet.