The FlightGear flight simulator project is an open-source, multi-platform, cooperative flight simulator development project. It also simulates space flight in Earth's orbit.

FlightGear's flight dynamics engine (JSBSim) is used in a 2015 NASA benchmark to judge new simulation code to the standards of the space industry. It's used professionally for research and development as well as non-professionally.

Wiki entry on FlightGear.

Tours of real locations inside the game.

FlightGear wiki.

FlightGear 2020.3.8 and later has 3d models of buildings, roads, cities etc. , based on Open Streetmap (OSM) data and automatic generation, for the whole world.