FlightGear is an open-source multi-platform flight simulator.
Ranked in these QuestionsQuestion Ranking
Pro Free and Open Source
All code written for FlightGear is opensource and available for anyone to use.
Pro Worldwide Scenery
FlightGear has scenery that contains environments to fly in from the whole globe
Con Getting stuck upside down
After a crash a pilot may be stuck in an upside down position with no way to recover.