http://www.foxnews.com/story/0,2933,509804,00.html
"The best part of these applications — which require the second-generation iPod Touch that came out last year — is that they are free to download, and calls to other people using the same app won't cost you anything.
Two of the services I've tried, Truphone and Fring, will also let you make free calls to Google Talk users and type instant messages to friends online."