FlightGear Flight Simulator project is an open-source, multi-platform, cooperative flight simulator development project. Source code for the entire project is available and licensed under the GNU General Public License.
The goal of the FlightGear project is to create a sophisticated flight simulator framework for use in research or academic environments, for the development and pursuit of other interesting flight simulation ideas, and as an end-user application. We are developing a sophisticated, open simulation framework that can be expanded and improved upon by anyone interested in contributing.
There are many exciting possibilities for an open, free flight sim. We hope that this project will be interesting and useful to many people in many areas.
FlightGear comes with a set of illustrated documentation, notably "The Manual", which is available as PDF and as HTML for online-viewing; these documents are listed on this site: http://www.flightgear.org/docs.html If you prefer to follow the 'bleeding edge' set of FlightGear instructions, then the following articles are likely to make you happy. Parts of this Wiki duplicate information that's alread present in "The Manual".