MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
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