Open Whisper Systems / ChatSecure
Open Whisper Systems provides open source, secure, encrypted mobile communication and storage tools. The Redphone app for Android, and now Signal for iOS, provide end-to-end encryption for mobile phone calls. TextSecure for Android encrypts text and chat messages over the air and on the phone. Open Whisper Systems has also introduced Flock, a private cloud sync service for Android contacts and calendar.
ChatSecure is a free and open source encrypted chat client for iPhone and Android that supports OTR encryption over XMPP. ChatSecure was originally available for only iOS devices, but is now also available on Android. With these tools, both Android and iOS users can communicate and share (more) securely on popular hardware platforms.