VoIPstudio
VoIPstudio is an award-winning VoIP system that offers businesses of all sizes a scalable communication solution. It’s inexpensive, easy-to-use, offers a full suite of PBX features, and is fully integrated for CRM.
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VoIPstudio
Kopete
Empathy
Psi-IM
Gajim
IM+
Trillian
Miranda NG
Adium
PhonerLite
Tragofone
Lingo
tSIP
YakYak
Jami
SessionTalk SIP Softphone
Phoner
KopeteEmpathy - Apps/Empathy - GNOME Wiki!
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
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
tSIP - tomeko.net/software: SIP client for Windows.