This app has been endorsed by Edward Snowden -whistle blowers and privacy advocate. In his words
Use anything by Open Whisper Systems.
A desktop version has now been released for Signal App through chrome browser. This app was in the beta stated previously for android and iOS.
It is required for a user to first install the android or iPhone version depending on the platform you are using first. Once that is done, you can go and and download the chrome extension of the app and scan the QR code from your mobile version, and then your account is linked with that of the mobile.
Signal App is free, and has a secured end-to-end encryption, your conversations are private, and it only works with existing phone numbers.
You can download the app directly via chrome browser here.