tSIP - tomeko.net/software: SIP client for Windows.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
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.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems