From The Intercept: You Should Really Consider Installing Signal, an Encrypted Messaging App for iPhone.
App maker Open Whisper Systems took an important step in this direction today with the release of a major new version of its Signal encrypted calling app for iPhones and iPads. The new version, Signal 2.0, folds in support for encrypted text messages using a protocol called TextSecure, meaning users can communicate using voice and text while remaining confident nothing can be intercepted in transit over the internet.
That may not sound like a particularly big deal, given that other encrypted communication apps are available for iOS, but Signal 2.0 offers something tremendously useful: peace of mind.
Unlike other text messaging products, Signal’s code is open source, meaning it can be inspected by experts, and the app also supports forward secrecy, so if an attacker steals your encryption key, they cannot go back and decrypt messages they may have collected in the past. [continue]
By the way: if you happen to be an Android user, check out the Wickr program.