If anyone is old enough to remember Chris Roberts famous Wing Commander series back in the 90s here's a treat for you. A modding team has spent many years modding the Freespace 2 engine to create a space flight simulator that is inspired from Wing Commander 3 in every way.
This release, Wing Commander Saga can be found at this link and is free to download and play, is entirely standalone, requiring no other programs/games to run. My only recommendation is having a joystick, cause flying with a keyboard is no fun :D
http://wcsaga.com/