Viber is a new free VoIP app for the iPhone. It allows you to talk using the AT&T data network, rather than the cellular network. I kept looking for a catch, but there isn't one. Works on WiFi and 3G. Check. You don't need to create a friends list. Check. Alerts you to incoming calls without the app being open. Check. No ads. Check.
International and domestic calls between 2 Viber equipped iPhones are free. You aren't burning your voice plan minutes, and soon there will be an Android version. Of course, if you have a metered data plan, you may have to watch out to make sure you don't exceed the limit, but many long-time iPhone users are still hanging on to their unlimited data contracts.
In my tests, voice audio quality is terrific. It makes you realize how bad the AT&T wireless voice audio quality really is. Registration for Viber is completely pain free: you give Viber your phone number, and then they text you with an access code. You enter the access code into the app, and then you are on your way.
The Viber interface looks just like the built-in iPhone dialer. You can see your regular contact list; the difference is that friends who are on Viber are marked so you know who you can call for free. You can even call out to non-Viber equipped phones, but you'll use your regular minutes.