FlightGear is a free flight simulator. And simulator really is the word here. You can fly real planes over real locations with a crazy amount of detail. (Just look at those control panels!) It's even meant to be used in research or for pilot training. The amount of content is also crazy; FlightGear sports over 500 aircraft and scenery from all over the world. There is also multiplayer, scripting and some impressive graphics. Intrigued? I just got, so I'm downloading now. :)
Impressive landscapes stretches far and wide.
And here's a shot of those intstrument panels mentioned above. :)