Jitsi

Jitsi (previously SIP Communicator) is an audio/video and chat communicator that supports protocols such as SIP, XMPP/Jabber, AIM/ICQ, Windows Live, Yahoo!, Bonjour and many other useful features.

Jitsi supports the ZRTP protocol stack by Phil Zimmerman for encrypted private communications.
Jitsi is Open Source / Free Software, and is available under the terms of the LGPL and performs Secure video calls, conferencing, chat, desktop sharing, file transfer, support for your favorite OS, and IM network. All this, and more, in Jitsi - the most complete and advanced open source communicator.