X-Plane is quite possibly the most realistic and flight simulator available to the masses. Compatible with Windows, Mac and Linux it is by far, in my opinion the most diverse flight simulator created. You can fly a multitude of aircraft and can design your own aircraft, scenery and flight plans. As a real pilot myself, I find X-Plane excellent for sharpening instrument flying and also superb for getting to know unfamiliar airfields. I also enjoy experimenting, creating my own designs as well as recreating aircraft from the real world. If you haven't already, grab the demo and try it, its fantastic.