FlightGear

FlightGear is a free open-source flight simulator. It supports a variety of popular platforms (Windows, Mac, Linux, etc.) and is developed by skilled volunteers from around the world since 1996. 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 and open flight simulator framework for use in research or academic environments, pilot training, as an industry engineering tool, for DIY-ers to pursue their favourite interesting flight simulation idea, and last but certainly not least as a fun, realistic, and challenging desktop flight simulator. We are developing a sophisticated, open simulation framework that can be expanded and improved upon by anyone interested in contributing. 

If you like to support the project financially, you can make an entirely optional donation. Your donation helps us cover infrastructural expenses, such as for servers, CDNs and domain registrations.