Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
tSIP - tomeko.net/software: SIP client for Windows.
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.
Jami - Application and Data, Collaboration, and Web and Video Conferencing
Phoner - Softphone for VoIP, CAPI und TAPI.