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