FlightGear is a completely open-source and multi-platform flight simulator. X-Plane by Laminar Research is an open, customizable and multi-platform flight simulator. Also check out the X-Plane community. Then there is VATSIM – Virtual Air Traffic Simulation Network – a unique website aimed at pilot and air traffic controller training. Someone on Google’s dev team obviously had a lot of free time… and probably worked for a flight simulation company earlier. Easter eggs don’t get any better than this. Update 2015-03-29: Big Fat Simulations offers a range of interesting Air Traffic Controller Simulations.