FlightGear is a flight simulator compatible with virtually any operating system including Windows, Mac and Linux - with several unique features and options that may be quite different than you're used to in a simulation application.
Graphics are very close in quality to its competitor's and the performance is superb, faithful to the developers' stated intention that eye candy should not compromise performance. Therefore FlightGear graphics aren't quite as glitzy as the competition but the performance gain more than makes up for it; flights are smooth with no hesitation and there's even a landable aircraft carrier off the California coast. There are a few minor weaknesses such as ground handling and less than perfect audio feedback. The simulator doesn't have collision detection so you might land just fine on water without pontoons!
All in all, FlightGear packs in a lot of extras you'll not find in other simulators and since development is ongoing there's usually something new to enhance this one-of-a-kind experience.
The FlightGear flight simulator project is an open-source, multi-platform, cooperative flight simulator development project.