If you think that flight simulators are a dying breed then you have missed out on the rise of FlightGear, an open source flight simulator started in 1997 that can be regarded as a game or as a research tool.
The latest version 2.4 has just been released and it has some significant improvements. As always what you consider important depends on your interests, but notable is the overhauled weather module which now sounds more like a weather prediction/simulation system. You can set up some initial weather using METAR reports and the system will generate weather based on the state of the atmosphere and the local terrain. This can result in fog, thermals and topographic clouds. Clearly fun if you are flying a glider and taking the weather into account is now an essential part of the experience.
Graphical improvements include better rendering of mountains, water movement and reflections. The latest aircraft models are so good you can see yourself reflected in the bodywork. There is also a 3D rendering option which provides a good reason to go out and buy some new hardware. The experience of flying has been improved with better autopilot control, a new heads up display and many new cockpit systems.
A new HLA (High Level Architecture) interface allows communication between a number of simulations running on separate hardware. In the future this should allow the simulator to interact with commercial flight simulators.
You can also now opt for scenery to be automatically updated by download. You can also select any of 500 aircraft. The system also includes an AI module that can generate flight plans and control the trajectory of other flying objects and simulate flight traffic.
If you would like to see the new version in action here is a video of Harrier landing on some bridge or other...
The source code is available for download and binaries are available for Windows, MacOS 10.5-10.7, Linux and more.