Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
GlobalMeet Collaboration - From cloud PBX to conference calls and meetings to online events, GlobalMeet is a business communications platform that helps take you where you need to go.
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs