Psi-IM - Psi-IM is a messaging program that is designed for the XMPP network.
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.
Gajim - Full featured and easy to use Jabber client
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Empathy - Apps/Empathy - GNOME Wiki!
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.