Jitsi is an open source alternative to Skype

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 lets you connect to Facebook, GoogleTalk, XMPP, Windows Live, Yahoo!, AIM, and ICQ so that you can chat to all your friends in the simplest possible way.