Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
Crypho - End-to-end-encrypted messaging and file sharing
Keybase Chat - End-to-end encrypted chat built into Keybase
blockslack.io - Chat for small teams — encrypted and decentralized
Wire by EY - A highly secured collaboration platform
dPhone - The most secure calling app. Open Source and Free.