Viber, which was originally called Viber - Free Phone Calls, is an application which is similar to Skype. With it, you can send text messages and make phone calls for free, so long as you have an Internet connection.
Thanks to Viber, you can share text messages, photographs, messages with videos or voice, locations, emoticons, and tags. In fact, within the app, you can find a sticker store where you can find new tags to buy and use in conversations.
Once you install the app on your device, all you have to do is let it access your contact list, and in a matter of seconds you can start calling with pretty good audio quality and no expenses except the Internet service. It's better if this connection is WiFi or 3G, since using this app with a data plan can be more expensive than using the phone.
A positive element is that besides the mobile app, it has a client for Windows and Mac. Thanks to this, you can continue your conversations on your computer, making and receiving calls no matter where you are.
Viber is a good alternative in the wide range of instant messaging apps for Android. Its audio quality during calls is pretty good by general standards.