WhatsApp is a leading non-SMS messaging app that's available on several platforms but has seen dramatic success on Android with over 500 million app installs. The app lets you send messages with text, pictures and video, and also has the ability for group messages, all without any carrier charges or involvement. WhatsApp ties into your contacts list to keep it up to date at all times without the need to add or edit friends manually.
One of its biggest draws (and growth areas) is that WhatsApp works internationally without a hitch. Its business model is admittedly quite odd, charging users just $0.99 per year, and even giving the first year's subscription up for free.
Facebook acquired WhatsApp in February 2014 for over $16 billion, but the app has since operated independently with little or no intervention from the new parent company. It continues to be one of the most popular messaging apps in the world.