FlightGear is an open-source flight simulator. It supports a variety of popular platforms (Windows, Mac, Linux, etc.) and is developed by skilled volunteers from around the world. Source code for the entire project is available and licensed under the GNU General Public License. FlightGear has been used for desktop simulation, engineering simulations, FAA certified simulators, university research, and the aerospace industry. You will need to turn on TerraSync to automatically download scenery.