freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
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.
Mitel - Business Communications and Collaboration
Asterisk - An open source telephony switching and private branch exchange service for Linux.
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.