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.
Taptalk - Personal video and photo messaging
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS
GroupMe - GroupMe is a mobile group messaging app owned by Microsoft.
Snapchat - Real-time picture chatting
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.