Continuing the discussion from Alternative to Skype and facetime:
Jitsi is by far the best Skype alternative I have come across. See: Jitsi
Why? It features encryption for IM and VoIP and it works out of the box. Off-the-Record Messaging (OTR) for instant messaging conversations and ZRTP for VoIP telephony. I know of no other open source project that is as secure, feature-rich and easy to use. It just seems that they have stopped offering their XMPP service with which I have had good experiences. It would be fantastic to get it included in the Software Boutique.
Installation and security guide from security in-a-box: Jitsi installation and improved security settings