How's FlightGear? For example, is that god-awful HUD a code limitation, or do they just not care much about realism (doesn't seem likely, considering the "mission statement" on their front page)?
It depends. For a free open source flight simulator, FlightGear is good though the graphics aren't as good as FSX from what I've read. FlightGear is dependent on volunteer contributions . The aircraft and scenery are open source and differ.
For example, an A-6:
Yellow indicates the click able panels you can click on.