Jitsi – Secure, simple and scalable multiuser video conferencing client

Jitsi (AKA: Jitsi Desktop, formerly called SIP Communicator) is a powerful, scalable, secure (using the SIP/RTP/SRTP/ZRTP encryption algorithms), high-quality and community-driven VoIP/video conferencing/instant messaging application from France, written in Java. It's free, open-source, cross-platform (across Windows, Linux, Mac OS X desktop platfroms and Andrjoid, iOS mobile devices), multi-protocol and multi-user on use and allows to be made secondary development.