What you want is Orbiter. It's the best one I've seen. It's
a complete space simulation, however it's practically limited
to the solar system. It has massive number of addons and
mods.
Even though it's just the solar system (you can go further) it
works exactly the same as real current ships operate. In other
words, you will need to have some understanding of orbital
mechanics, even a basic understanding will help. Getting around
is not as easy as movies make you think, it's quite difficult. However
it's so satisfying to leave the earth, setup a nice orbit, and a good
ejection burn to go to Mars, and aim to aerobrake in the Mars atmosphere,
and pull it off just as you wanted. Definitely for the serious space nut,
and those who want to know more. Very satisfying experience.
Google Orbiter n u'll find it.
Another is Celestia, though it doesn't have the detail of Orbiter,
it does allow full exploration of the galaxy, has masses of addons,
and allows you to travel at any speed, including light speed or higher.
One thing I learned from it was in the scale of even the solar system,
Light speed is rather slow. Using it to just get to the nearest star takes
ages, probably days (I usually gave up n hit multiple levels of light speed. lol
Google Celestia for that. You can't land on planets though.