Asterisk - An open source telephony switching and private branch exchange service for Linux.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.
Melrose Labs CloudPBX - Advanced Hosted Cloud PBX for Your Team.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.