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
tSIP - tomeko.net/software: SIP client for Windows.
YakYak - Desktop client for Google Hangouts
Jami - Application and Data, Collaboration, and Web and Video Conferencing
CounterPath Bria - VoIP and video calling, messaging and presence - all from your computer desktop with the award-winning Bria 5 softphone client.