MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Jami - Application and Data, Collaboration, and Web and Video Conferencing
tSIP - tomeko.net/software: SIP client for Windows.
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).