tSIP - tomeko.net/software: SIP client for Windows.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Telephone - Telephone is a VoIP program which allows you to make phone calls over the internet.
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.