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.
Kopete - Kopete is an instant messenger supporting AIM, ICQ, MSN, Yahoo, Jabber, IRC, Gadu-Gadu, Novell...