It only took Edward Snowden to post a couple of tweets to turn Signal Private Messenger into the new favorite instant messaging application for security paranoid.
It is a tool to be able to chat from you to you or in a group, with the particularity that our communications are encrypted from end to end, so that nothing and nobody can spy on our conversations, not even the happy NSA. Same as WhatsApp.
Signal functions are not new, but integrate the best of RedPhone and TextSecure, two applications of the same company Open Whisper Systems. And as it could not be otherwise, it is also an open-source development.