Asterisk - An open source telephony switching and private branch exchange service for Linux.
Tragofone - White-label VoIP Softphone with WebRTC and Auto-provisioning
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.
YakYak - Desktop client for Google Hangouts
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
PhonerLite - PhonerLite enables your PC to use it for internet telephony (VoIP , Voice over IP, SIP).