Empathy - Apps/Empathy - GNOME Wiki!
TollFreeForwarding - TollFreeForwarding offers hosted PBX, call forwarding and IVR solutions.
Psi-IM - Psi-IM is a messaging program that is designed for the XMPP network.
Loop Communications - Loop Communications provides hosted VoIP business phone systems to small businesses and mid-sized companies.
Gajim - Full featured and easy to use Jabber client
Vonage - Vonage is a cloud-based all-in-one VoIP software solution designed for multiple industries such as accounting, healthcare, finance and marketing etc.