You can download skype.com .You can also make inte rnational calls using skype. and make calls to others who also download skype.
. Or else you can purchase magic jack.
http://www.magicjack.com/plus-v05/. You can also purchase it from radio shack or qvc, for $19.99
Google voice is one. This next link list a bunch of them . Scroll down to end of the article.