FlightGear is an 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. Source code for the entire project is available and licensed under the GNU General Public License.
For Windows, Linux and Macintosh. Get the Linux distro at the official site.
If you like the game and want to support the authors, consider buying the DVD of the game or others merchandise at the official site store.
FlightGear v220.127.116.11 Windows
515.5 MiB - 287 downloads
FlightGear v2.6.0 Macintosh
598.4 MiB - 126 downloads
FlightGear AirCraft Pack 2.6
2.0 GiB - 109 downloads
FlightGear Scenery Pack
522.2 MiB - 91 downloads
Single add-ons can be downloaded on the official Site.