Chrome: Signal, the encrypted chat app built by Open Whisper Systems and approved by the likes of Edward Snowden, is now available as a desktop app through Chrome. Unlike a traditional desktop app, Signal requires that you first install the Android or iPhone version of the app. Once one of those is installed, grab the Chrome version, scan a QR code from the mobile version and then your accounts are linked from there. As you'd expect, the desktop version works the same as the mobile version, which means it's free, includes end-to-end encryption and works with your existing phone number.
Signal Private Messenger (Free) [Chrome Web Store]