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Jitsi

Jitsi (previously SIP Communicator) is an audio/video and chat communicator with full end-to-end encryption that supports protocols such as SIP, XMPP/Jabber, ICQ/AIM, Windows Live, Yahoo!, GTalk/Hangouts extensions, as well as OTR, ZRTP, etc. It can handle every firewall, and has many other useful features. Jitsi is completely open source / free software, and is freely available under the terms of the GNU Lesser General Public License.