There's an open source multi platform sim available called 'Flightgear' & can be downloaded for free. From their website
www.flightgear.org:
"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."
The information given in the features page of the website is quite impressive -
www.flightgear.org/features
I've not yet installed it due lack of space but I intend to as soon as I can make room.