VATSIM is an organization which provides simulated air traffic control services. The services are provided through user developed extensions to the Microsoft Flight Simulator, user-developed controller workstations, and a network of servers. Using these services, a user piloting a virtual aircraft can file a flight plan and “fly” guided by other users who act as controllers. VATSIM has about 250,000 members who are virtual pilots and virtual controllers. Vatsim’s software is developed collaboratively by its members. Pilot and controller training is offered online.
Here’s a demonstration video: