Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.
Fongo - free VoIP app - free Canadian phone number
CSipSimple - CSipSimple is a SIP application for android devices.