International Virtual Aviation Organisation

International Virtual Aviation Organisation VZW (IVAO) is a non-profit association which operates a free-of-charge online flight-simulation network. Following free registration users can connect to the IVAO Network (IVAN) either as a virtual air traffic controller or as a virtual pilot and interact with each other in a large multiplayer environment utilising real-world aviation procedures, phraseology, and techniques.

Source: Wikipedia — International Virtual Aviation Organisation (CC BY-SA 4.0)