Wow, looks like some pretty impressive stuff is being done with Delta3D. Too bad some of it
doesn't appear readily available to civilians. (Granted, most of it's designed as training
aids rather than pure games, although no doubt there are plenty of us that would get a kick of
out of playing with the training versions.)
FlightGear (which somebody else mentioned above) is another package which has also been
incorporated into a number of other projects (either as the flight dynamics or the
visualization portion).
Those are great examples of what can be accomplished building on good engines with open
source, where you're not necessarily limited to whatever interface (if any) a proprietary
engine might offer.