Update November 4, 2015: The app is now called “Signal” on both Android and iOS.
If you use WhatsApp, Hangouts or a similar messaging app for Android or even if you just use plain SMS, please stop doing so and start using TextSecure now. TextSecure has all the same features, only everything is encrypted by default (so no one can snoop on your messages), it is Free Software (the source code is available and can be audited) and it is a pleasure to use.
It seamlessly replaces the standard SMS app, switching all communications between TextSecure users automatically to the secure system, incidentally also saving them the SMS fee. So, if you care a bit about privacy (and even if you don’t), install this app now. It literally costs nothing, doesn’t require you to change anything and makes everyone better off.