I posted this earlier, but WordPress decided to act up on me.
The author of X-Plane has created an add-on to let you fly the space shuttle as it comes back in for landing. The author also wrote up his experience on one of these (simulated) flights.
In the same vein as the orbiter landing, he also got enough detailed information from various NASA sites to reproduce a Mars flight sim, also available as an add-on to X-Plane.
Both are quite old, dating back to February of 2000 but are interesting none-the-less.