Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Asterisk - An open source telephony switching and private branch exchange service for Linux.
LiveAgent - LiveAgent is a fully-featured web-based live chat and help desk software. It harnesses the power of a universal inbox, real-time live chat, built-in call center, and a robust customer service portal. Start your free 1 month trial today!
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.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.