PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Jami - Application and Data, Collaboration, and Web and Video Conferencing
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
tSIP - tomeko.net/software: SIP client for Windows.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs