In my experience skype has been the best so far. Both voice and video call is free to use from one computer to another as long as it is one on one. As
far as i know from anywhere in the world to where ever you want to call.
I'm in a remote area in europe and frequently use both skype voice and video with my co-workers who are in the states and other places worldwide. It
has never cost me a dime.
This link should tell you a bit about what is free and what is not when it comes to skype.