Jitsi

Jitsi (formerly 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. It is currently the most feature rich Internet Communicator, and is the only one to provide encrypted audio/video calls, desktop streaming and sharing, and chat on Windows, Mac OS X, and Linux with both the SIP and XMPP protocols.