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).
YakYak - Desktop client for Google Hangouts
SessionTalk SIP Softphone - SessionTalk Desktop Softphone is a feature rich SIP Client for Windows devices with Video, Messaging, Push Notifications, Encryption, Dial Plan, Call Transfer and Call Recording.
Phoner - Softphone for VoIP, CAPI und TAPI.
Jami - Application and Data, Collaboration, and Web and Video Conferencing