Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Keybase Chat - End-to-end encrypted chat built into Keybase
StealthChat - StealthChat is Encrypted, Private, Secure Calls and Chats on Android and Apple iOS
Crypho - End-to-end-encrypted messaging and file sharing
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.