FlightGear Flight Simulator

Open-source provides another great piece of work. If you have ever wondered about OSS flight simulators, this one definitely takes the cake.
From flying an Airbus A380, to a B52-F Stratofortress, FlightGear has it all. This piece of ingenius design work grew out of many flight sim fans dissatisfaction with current commercial flight simulators.
View the screenshots here and pee your pants in envy. Then approach the downloads here and get to taking off into the wild blue yonder.
FlightGear Flight Simulator is open-source, and is supported in linux, Windows, Solaris, SGI, MacOSX and FreeBSD.