tSIP - tomeko.net/software: SIP client for Windows.
Telephone - Telephone is a VoIP program which allows you to make phone calls over the internet.
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.
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.
Acrobits Softphone - Acrobits Softphone, an app for entry into the world of SIP.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.