Viber, a free VoIP app for the iPhone was recently launched.
Although there are already plenty of mobile VoIP apps for the iPhone, what makes Viber different, is that it does not use buddy lists or require registration. Instead, your phone number is your username. Your friends also have to be Viber users in order for it to work, but once they join all calls to them are free. Another nice thing about the app is that it can run in the background and receive phone calls. However, even if you don’t have it running, you can still receive a push notification, click ‘Answer’, and then the app is launched to accept the call. The app supports Bluetooth headsets as well.
Viber is currently working on an Android version.