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.
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.
CoyIM - CoyIM is a safe and secure chat client.
Tencent QQ - Tencent QQ is a free instant messaging computer program developed by the Chinese company Tencent Holdings Limited.
Empathy - Apps/Empathy - GNOME Wiki!