“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.”

This free flight simulator is available in Windows, Sgi Irix, several Linux, Solaris, and Mac OS 10.2 flavors (some OS ports are still at version 0.9.2). If you can’t download it from the main site, try one of the mirrors.