http://orbit.medphys.ucl.ac.uk/
You can fly various ships including the shuttle Atlantis, and fly scenarios that include moon shots and docking with the ISS.
The learning curve is quite high. You actually have to learn some orbital mechanics, and some math to even achieve orbit.
I spent over a month flying smaller craft, learning the ropes, before I could even think of trying to get the Shuttle into orbit.
I finally learned enough to try the Shuttle. Here are some screen shots from one of my Shuttle Atlantis flights to the ISS.
