VATSIM, or Virtual Air Traffic Simulation network is a free online platform that allows virtual aviation enthusiasts using various flight simulation software to connect their simulator of choice to one shared virtual world.
The main focus of VATSIM is providing a convincing and immersive simulation of controlled airspace. You can read more about VATSIM in the official documentation.
VATSIM Flight Tracker allows for displaying currently logged-in pilots and controllers as well as some general statistics about the network.