Telephony - Free Software Directory - Free Software Foundation
Coccinella
Ekiga.net (already mentioned at the forum and is also available for windows users)
Ekiga ~ Free Your Speech
Linphone
QuteCom
Twinkle
Why not Skype?
Free software replacement for Skype
Skype is a proprietary Voice-over-IP program that uses a proprietary protocol. Skype is seducing free software users into using proprietary software, often two users at a time. Using proprietary phone software means that we can't be sure who is listening in, because we can't see the code. The Chinese government, for example, was found to have been spying on Skype conversations already, and they are probably not the only ones. We do not want to encourage the creation of a Skype compatible client, but instead, we want to encourage you to create, contribute to, or promote the use of free software replacements for Skype, such as Ekiga, and to encourage adoption and use of free VoIP, video, and chat protocols such as SIP and XMPP/Jingle.