tSIP - tomeko.net/software: SIP client for Windows.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Telephone - Telephone is a VoIP program which allows you to make phone calls over the internet.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
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.