Download WhatsApp Messenger for Android, iOS, Blackberry, WP8
WhatsApp is a cross-platform instant messaging service that exploded in popularity, with more than a billion users exchanging messages, photos and calls every day. Ease of use and contacts setup, cross platform functionality and media-rich messaging are some of the app's major selling points, and the app continues to get better with a new Web app and a transition to a totally free model for its end users. Check out our rundown of how to set up and use WhatsApp, as well as a few tips and tricks to make it easier to use.
Screenshots show the Android version of WhatsApp but include instructions for multiple OSs where appropriate.