PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Jami - Application and Data, Collaboration, and Web and Video Conferencing
Lingo - Empower Creation and Ensure Brand Consistency. Build and Share libraries of visual assets with your entire team — all from your desktop.
tSIP - tomeko.net/software: SIP client for Windows.
YakYak - Desktop client for Google Hangouts