Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Empathy - Apps/Empathy - GNOME Wiki!
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Psi-IM - Psi-IM is a messaging program that is designed for the XMPP network.
Lingo - Empower Creation and Ensure Brand Consistency. Build and Share libraries of visual assets with your entire team — all from your desktop.
Gajim - Full featured and easy to use Jabber client