Keybase

Keybase is not so much a messaging application as it is messaging platform since it can interface with so many other platforms. Furthermore, users don’t need to know someone else's number or email in order to contact him.
 
It’s important to mention that all messages are secure and use end-to-end encryption, and they can survive and transfer, with encryption, to new phones and platforms.
 
Furthermore, Keybase is a completely free application that’s open source, and that’s work on lots of other platforms, including Mac OS and Linux.