Fighter Squadron: The Screamin' Demons over Europe

Fighter Squadron: The Screamin' Demons Over Europe is a WWII flight simulator where you can fly a selection of ten different US, British and German fighters and bombers in a series of 30 missions over Dover, the heart of Germany, or North Africa. You can play the missions for either side, man any of the aircraft, as well as change positions inside the bombers to man the guns and bombsight.

It also allows the users to add their own planes, vehicles and missions via a text-based plug-in format called OpenPlane. Every part of the aircraft is individually listed in its OpenPlane file and can be edited - you can change the power of the engines for instance, or the distance the various gun turrets can traverse.