VoipStunt
Free phone calls with voip stunt. call your online friends for free as well as a special selection of popular international destinations.
http://www.voipstunt.com/en/index.html
VoipBuster
VoipBuster
http://www.voipbuster.com/nl/splashvoipin.html
Sipphone
Gizmo project provides low cost international calling and free calls to users on yahoo messenger, google talk, windows live users and sip networks.
http://www.sipphone.com/
Skype
Download skype and calls other people using skype for nothing - wherever they are in the world.
http://www.skype.com/