Psi-IM - Psi-IM is a messaging program that is designed for the XMPP network.
Empathy - Apps/Empathy - GNOME Wiki!
Gajim - Full featured and easy to use Jabber client
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
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.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...