Taken from the official site
The Flight Gear Flight Simulator project is a free, open-source, multi-platform, cooperative flight sim development project. Source code for the entire project is available and licensed under the GPL.
The Flight Gear project is working to create a sophisticated flight simulator framework for the development and pursuit of interesting flight simulator ideas. We are developing a solid basic sim that can be expanded and improved upon by anyone interested in contributing.
There are many exciting possibilties for an open, free flight sim. We hope that this project will be interesting and useful to many people in many areas.