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.
X-Plane is a flight simulator that reads in the geometric shape of any aircraft and then figures out how that aircraft will fly. It does this via an engineering process called "blade element theory", which involves breaking the aircraft down into many small elements and then finding the forces on each little element many times per second. These forces are then converted into accelerations, which are then integrated to velocities and positions. This gives X-Plane the most realistic flight model available for personal computers.
JSBSim is an object-oriented (C++), multi-platform, 6-DOF, Flight Dynamics Model (FDM). It can be run as a standalone, batch-mode flight simulator (no graphical displays), or integrated with FlightGear or other flight simulator.
IFT is a small flight simulator written for the purpose of training pilots not experienced in instrument flight. It includes a couple of VOR and NDB stations and displays. These can be used to exercise basic radio navigation skills.
It's made and tested on GNU/Linux, but it should work on all Unices, since all it uses is the X11 library.