WhatsApp is a hugely popular instant messaging (IM) app that lets you send text image, video, or audio to your friends, colleagues, and family over Wi-Fi and cellular data networks (rather than the SMS/MMS network). WhatsApp supports both direct and group messaging and read state. Accounts are setup using your phone number as the only identifier, and there are no ads, in-app purchases, or other charges. The first year is free and additional years are $1. Simple as that.

Originally setup as a cross-platform clone of BlackBerry Messenger (BBM) that could be used on almost any type of internet-connected phone imaginable, WhatsApp grew to become one of the biggest instant messaging networks on the planet. WhatsApp was acquired by Facebook in February of 2014 for $16 million dollars.