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.
OpenUCF is a unified communications/VoIP framework for PHP5 supporting telephony, presence, and other communication-related features. It has a Web interface for users and administrators. It features a RESTful API, rich-presence, VoIP/SIP based on Asterisk, and XMPP presence based on jabberd2.