The Taridium ipbx is a complete software-based VoIP PBX system that replaces a traditional proprietary hardware PBX. It runs on Linux without the need for extra software licenses, and is based on the SIP standard. It supports many phones and is available for free in a 5 user version.
Gemeinschaft is a PBX based on Asterisk, MySQL, Apache, and PHP, and designed for high availability and clustering. It provides automatic provisioning for mass deployment, and can handle over 10,000 users. Administration is done via shell scripts or a Web GUI. Hot-desking and mobility are supported. German voice prompts are included. There is a standards-compliant Web GUI with internationalization (and German and English translations). Outbound and inbound routing with full PCRE support is included.
Sipcat is a software IP telephony system that helps businesses increase productivity and reduce telecommunication costs. It allows you to start running your own voice over IP PBX in a matter of minutes. It contains a complete Linux distribution, including Asterisk and all required 3rd party software.
4PSA VoipNow is PBX software for service providers and enterprises. Easy to use and deploy, it can run on x86 or PowerPC based servers and delivers advanced IP telephony services. It can be deployed in various working scenarios to provide hosted VoIP services in high availability xSP environments, advanced call center services, or enterprise telephony in organizations of all sizes. A free Express version is available.
Sofia-SIP is a SIP user agent library, compliant with the IETF RFC3261 specification. It can be used as a building block for SIP client software for uses such as VoIP, IM, and many other real-time and person-to-person communication services. The primary target platform is GNU/Linux. Sofia-SIP is based on a SIP stack developed at the Nokia Research Center.
Gizmo Project is a program that uses advanced VoIP technology to allow you to talk with your friends around the world. Gizmo Project uses your Internet connection to make calls to people using other computers. It is simple, and allows you to talk clearly and for as long as you want.