tSIP - tomeko.net/software: SIP client for Windows.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
CommPeak PBX - CommPeak’s Hosted Cloud IP-PBX is a web-based highly customizable and feature-rich solution - the ideal, low-cost solution for an enterprise of all sizes.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Acrobits Softphone - Acrobits Softphone, an app for entry into the world of SIP.
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.