Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
tSIP - tomeko.net/software: SIP client for Windows.
YakYak - Desktop client for Google Hangouts
Jami - Application and Data, Collaboration, and Web and Video Conferencing
Phoner - Softphone for VoIP, CAPI und TAPI.