Threema

“Threema is a mobile messaging app that puts security first. With true end-to-end encryption, you can rest assured that only you and the intended recipient can read your messages.” Threema is my favourite instant messaging application and has been described as “a much flasher version of WhatsApp”. Its source code has recently undergone an external security audit and was found to provide a ”security level which compares favourably with the state of the art in similar messaging services“.
threema.ch