Gajim - Full featured and easy to use Jabber client
CoyIM - CoyIM is a safe and secure chat client.
Palringo - Palringo is a cross – platform instant messaging client with which you can communicate with your contacts from MSN, Gmail, Yahoo MSN, AOL, Jabber or ICQ at the same time.
Empathy - Apps/Empathy - GNOME Wiki!
Dino - Dino is a modern open-source chat client for the desktop.
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.