tSIP - tomeko.net/software: SIP client for Windows.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
PBXware - PBXware is the world’s first and most mature IP PBX Professional Open Standards Turnkey Telephony Platform. Bicom Systems’ softswitch is offered in the Business, Call Center and Multi-Tenant Editions each supporting specific features.
CommPeak PBX - CommPeak’s Hosted Cloud IP-PBX is a web-based highly customizable and feature-rich solution - the ideal, low-cost solution for an enterprise of all sizes.
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Acrobits Softphone - Acrobits Softphone, an app for entry into the world of SIP.