If you are someone who loves talking to friends over your iPhone, here is an application that you would love. Viber is a recently launched app that helps to reduce your monthly bills by letting you call your friends over VoIP. But unlike Skype, you do not have to register with an account to start using Viber. The application automatically scans your address book to compile a list of contacts who have Viber installed on their iDevices. This enables you to call these contacts for free using Viber.
While the app works over Wi-Fi as well as 3G networks, do note that you may end up with high monthly bills unless you have a 3G-friendly data plan. The application is presently available only on the Apple App Store though an alternate version for Android and Blackberry platforms are soon on the way.