Empathy - Apps/Empathy - GNOME Wiki!
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Psi-IM - Psi-IM is a messaging program that is designed for the XMPP network.
Gajim - Full featured and easy to use Jabber client
Pinger - Pinger lets you text and talk free with the people you care about most, anywhere on the planet.
Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.