OpenWengo:
SIP compliant VoIP client (+ Video ?), Firefox extension, GPL: http://openwengo.com/

Ekiga:
VoIP and video conferencing application (H.323 and SIP) for GNOME, GPL: http://www.ekiga.org/

PhoneGaim:
VoIP system based on the Gaim instant messaging software and the SIP protocol, GPL: http://www.phonegaim.com/ (sponsored by Linspire: Some controversy exists over the perceived difficulty faced by non-Linspire users who wish to install PhoneGaim on their Linux system.)

sipX:
SIP based products (pbx, softphone, and supporting products), LGPL: http://www.sipfoundry.org/sipX/

Google's Libjingle:
Implementation of Jingle and Jingle-Audio (proposed extensions to XMPP), BSD License: https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fsourceforge.net%2Fprojects%2Flibjingle%2F

KPhone:
VoIP application for the KDE desktop environment, SIP, GPL: http://kphone.sourceforge.net/


Skype: http://www.skype.com/

Google Talk: http://www.google.com/talk/

Gizmo Project: http://gizmoproject.com/