The 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.

WWW: http://www.flightgear.org/