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.
Ding is a PHP framework that provides dependency injection (by Setter, Constructor, and Method), Aspect Oriented Programming, XML, YAML, Events support, and some JSR 250/330 annotations as bean definition providers, lightweight, can be deployed as a PHAR file, simple, and quick MVC, syslog, TCP client and server with non-blocking sockets, timers, and custom error, signal, and exception handling, PAGI integration (for the Asterisk gateway interface), and PAMI integration (for Asterisk management). It is similar to Java's Seasar and Spring.
Shift8 provides a simple mechanism for using the AJAM Asterisk interface in PHP. Apart from the obvious reason of providing an easy way to talk with a remote Asterisk server, the main benefit of having an intermediate library between your software and an Asterisk server is mostly that you can isolate the remote Asterisk, protecting the credentials and the entire PBX.
Asterisk Presence Panel is a simple application that allows the user to monitor the status of extensions on multiple Asterisk based PBX systems. The application connects to the Asterisk server using the manager interface. When it starts, it uses a manager command to probe the status of the extension, after which it relies on the extension status messages in the manager session to update its status. The application features contact groups, the ability to click to dial a contact, computer driven dialling for any number, the ability to connect to multiple Asterisk servers, and integration with the system tray on supported platforms.