TextSecure from them has a lot of good people behind it (Moxie Marlinspike being one notable). It is open source, actively developed, and they have plans for an iOS and desktop client on top of the already existing Android client. They support both SMS and data for sending messages. It is aesthetically pleasing, simple to use, and supports group chat. I highly recommend it. Their blog has some interesting posts about some of the crypto its built on, if you're also interested in that stuff.