There's a couple. Check the app store and navigate there. Whistle seems to work fine for US calling, even if you have to listen to an ad for the first 15-20 seconds. Other than that, there's a guy around the Carfax tower that lets you dial anyone on his phone. No iPad app, but still, free calling
Whistle is ok, I used to use it before other options existed, and the ads they play can get annoying. Currently I use google voice tied into Talkatone (which is free to use), pretty awesome combo with no ads.