With more than two million downloads in 2009 alone, Asterisk is the oldest and most widely deployed open source telephony platform in the world. It was originally developed for Linux, but ports exist for Unix, Windows and other platforms, all free. And like any successful open source platform, there's a thriving development community (about 63,000 strong) that's constantly on the march to improve and broaden the platform.
Asterisk 1.8, released in July, included major enhancements. Among these is implementation of Secure RTP, a secure version of the Real-time Transport Protocol specification for delivery of audio and video packets and streams over IP. Version 1.8 integrates with CalDAV, iCal, Exchange and EWS calendaring systems, implements message waiting and other features using Jabber/XMPP/PubSub specs, and adds support for IPv6 under SIP.