A flight simulator open-source - FlightGear

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.
FlightGear is a free flight simulator project. It is being developed through the gracious contributions of source code and spare time by many talented people from around the globe. Among the many goals of this project are the quest to minimize short cuts and "do things right", the quest to learn and advance knowledge, and the quest to have better toys to play with.

Flight Geat 0.9.10 for Win (132.40MB)
Flight Geat 0.9.10 for Linux (1567.57)
Flight Geat 0.9.10 for Mac (165.20MB)

From: nothingtoxic.blogspot.com