FlightGear is a free, Open Source flight simulator in development since 1996 and features many additional planes, helicopters, and scenery updates you can download.
RASAero II is a combined aerodynamic analysis and flight simulation software package for model rockets and high power rockets, amateur rockets, and sounding rockets.
With X-Plane, you also get one of the most realistic sims on the market. It’s not free, but then again very few are and certainly nowhere as complete as this one.
Limit Load is a cockpit flight game featuring modern military jets. It is more an arcade game than a simulation, as the level of realism has been adjusted to provide action-packed gameplay.