The Viber VoIP and Texting app for Android is out of beta, and the full version is still free! The app basically allows you to use your 3G or WiFI connection for phone calls and texting, thereby saving your regular minutes. The primary advantage of this app is that it would allow you to make free international calls. Obviously, the person on the other end would also need to have the software on their phone. Viber is different than most VoIP services because it doesn’t require registration, and actually uses your own phone number. The app is also ad-free for now! It's available on the Market.

Source: AndroidCommunity