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
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.
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.