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.
FlightGear is a flight simulator that brings you freedom. Do what you want, how you want. Fly more than 400+ aircrafts, didnt like something in a particular aircraft? modify it easily, from textures to scripts and flight dynamics. Change everything to the way you like. Not other flight simulator gives you that freedom. By the way, its free! So grab it now!