Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
Wickr - Wickr is a free end-to-end encrypted text, video, picture and voice messenger with full sender...
ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
Burn Note - Burn Note is the name of a continuous private messaging utilizing which you can have an ongoing private discussion with your companions or any individual who is at Burn Note.
Keybase Chat - End-to-end encrypted chat built into Keybase
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.