ArmorText is a free Android app to secure text messages, it uses RSA1024 and AES256bit to encrypt your SMS&MMS messages, the receiver will need to have the same app installed to be able to decrypt the messages. ArmorText will connect to the Internet after launching it for the first time to retrieve your friends public key encryption. Security can easily be enabled tapping an ON/OFF lock button, a Smart Predict option will detect when the app believes you need to encrypt your text messages (based on the last texts sent) and automatically turn security on unless you decide otherwise, the app can stop message forwarding by the recipient too.
ArmorText is a pure text messaging solution, not a chat client, it only encrypts SMS and MMS messages with photos.
ArmorText Android SMS encryption
With smart phones increasingly used for mobile payments, email and online banking they have become a prized asset for thieves, ArmorText will protect your data even when it is not stored in your phone but the person you are communicating with, messages are encrypted before sending, stopping middle man eavesdroppers, like your network provider. Planned features for the future include controlling how many times a text message can be viewed, how long the message is available for and non-repudiation.
Update 2014: This app is no longer available in Google Play