Make Free Calls Online - Make Free Calls Online is a web application that, as the name suggests, allows its users to make international and local calls for free.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Poptox - Poptox is a web application where you can communicate with people in every corner of the world.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
IEvaphone - IEvaphone is a web service that can be used for connecting with friends and family overseas and for making important business calls providing its users the luxury to make a call to any place in the world with just a secure internet connection.
Sipdroid - Free and open-source SIP/VoIP Client