"VATSIM, or Virtual Air Traffic Simulation Network, is a non-profit organization operating a dedicated, worldwide, Internet-based flight-simulation network. Users can connect to the network to either fly online as a pilot using flight simulation software such as FSX, Prepar3D or X-Plane, or direct traffic as an air traffic controller (ATC) and participate in what has been described as a close approximation of real-life aviation procedures. The organization has achieved notability for its very high level of participation and realism, that has seen it in use as part of Federal Aviation Administration funded research projects. This notability resulted in the organization being featured on the front page of The Wall Street Journal in 2006. It is considered the largest online flight simulation network in the world with over 209,000 registered members as of September 3, 2009, with hundreds of participants online at any one time."
We strongly recommend to fly on the VATSIM network!
"IVAO or International Virtual Aviation Organisation VZW 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 engage and interact with each other in a massively multiplayer environment utilising real-world aviation procedures, phraseology and techniques."