Signal is arguably the most open, and certainly the most secure, privacy-preserving instant messaging/video or voice-calling system available today. It is open source, ad-free, standards-based, simple, and very well designed. Though not filled with bells and whistles, for most purposes it is a far better alternative to Facebook-owned WhatsApp or other near-competitors like Viber, FaceTime, Skype, etc, especially if you have any concerns about your privacy. Like all such things, Metcalfe's Law means its value increases with every new user added to the network. It's still at the low end of the uptake curve, but you can help to change that - get it now and tell your friends!

Like most others of its ilk it hooks into your cellphone number rather than a user name but, once you have installed it on your smartphone, you can associate that number (via a simple 2D barcode) with a desktop client. Until recently it only supported desktop machines via a Chrome browser (or equivalent - I used Vivaldi) but the new desktop clients are standalone, so you don't have to grind your system to a halt or share data with Google to install it. It is still a bit limited when it comes to audio (simple messaging only) and there still appears to be no video support (which is available on smartphone clients) but this is good progress.