I have a model with some objects above eache other and enable rigid body physics for them so the upper ones fall on the lower one. Now when I play the animation to a certain frame how can I put it as the default location/rotation of the mesh?
Select all your objects and hit bake to keyframes. Set the starting and endeding frames to the frame you want them to stay at. Go to that frame, and hit Alt-I. This will remove the unnecessary keyframe.
Alternatively, just click Apply Transformation while on the appropriate frame. You can then remove the rigid body constraint and the object will stay at that point.
Or, rather than removing the rigid body constraint, just change it from Active to Passive if you want other objects to be affected by it.