Qute Com
QuteCom (previously called WengoPhone) is a free software SIP compliant VoIP client developed by the QuteCom (previously OpenWengo) community under the GNU General Public License (GPL). It allows users to speak to other users of SIP compliant VoIP software at no cost. It also allows users to call landlines and cell phones, send SMS and make video calls. None of these functionalities are tied to a particular provider, allowing users to choose among any SIP provider.
Read more about Qute Com: History, Calls, User Interface, Features, Limitations