Cryptocat

Cryptocat is a cross-platform messaging application that is mostly focused on making sure that your privacy stays untouched while chatting or exchanging files with other users by relying on end-to-end encryption technologies.

Does not require installation and runs on all major OSes

The first thing you should know is the fact that this you can run this lightweight messaging utility on all major operating systems out there, namely Windows, macOS and Linux.
Getting started with the app is quite simple, since it does not require any type of installation. You just simply need to download the app's package, unzip it and launch the app via the designated executable file.

Requires that you register a Cryptocat account

Once launched, you should know that you can also rely on the icon taskbar to summon the app's main interface.
As expected, before you can start taking advantage of everything this program has to offer, you are required to register for an account and, subsequently, log in using the official credentials.

Simple interface with a typical layout

Right off the bat, it becomes evident that Cryptocat was not designed to win any beauty awards, although it sports a simplistic and very functional user interface. If you have ever used other messenger apps before, then it should be business as usual with this utility.
The main window encompasses a light menu bar, a simple search bar and displays the list with all your contacts or available devices. Double click on a device or a contact, and you can easily launch a private chat window.
The chat environment is also based on simplicity, with no distracting elements around, just simple chat-related features like a set of cat-like emoji and two buttons, one for sending files and the other for recording and sharing audio or video content.

No muss, no fuss instant messaging app that takes your privacy seriously

This said, it's quite clear that Cryptocat, with its straightforward, security-based philosophy, is aimed at users who do not place too much value on eye-catching interface elements or customization features but, instead,  simply want an app that's as less distracting as possible and that does the best at keeping their privacy safe.