PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
YakYak - Desktop client for Google Hangouts
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