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.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
The Incredible PBX - Incredible PBX is a secure and feature-rich implementation of the Asterisk® PBX.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
PBX in a Flash - PBX in a Flash is a PBX system based on Asterisk, FreePBX and CentOS, coming as an ISO download.