I actually learned of the app Whistle Phone here on the iPad forums. I decided to try the app and I must say it worked great from Europe. It will work using wifi or 3G connections and there is no charge to call the US. And, unlike Skype, you can call any number (mobile, landline, etc.) for free. Whistle also assigns a US based number to you so people in the US can call you too.
Apparently, the way Whistle provides this service for free is that occasionally, there is a 15 second ad that plays before your call is connected. Of the 20 calls I placed in Europe, I heard an ad 3 times so it was not at all inconvenient.