Getting an object lined up with the x, y, or z axis again I got a model from SketchUp, and imported it into Blender to modify it. It was just half the figure, meant in the end to be mirrored to have it be completed, and when I initially imported it into Blender it was lined up perfectly so that both the nose and tail were nicely aligned at x axis 0.

Through the course of working on it however I must have accidentally rotated it off center. I want to duplicate the left side so that I can mirror it and then merge these two halves, but I need to have that center loop of the figure uniformly on x axis 0. How do I get it squared up again?! See how it's been tilted off center?
Close the shape with ngon:


Add Empty to the scene, enable snapping, set to face, enable align orientation and snap the empty to the ngon:


Parent the mesh to the empty: Select mesh, select empty, Ctrl+P
Reset the rotation of empty with Alt+R
Clear parent (Alt+P) and Keep the transform. Now your mesh is aligned with world coords.
Delete Empty, delete ngon, rotate mesh to the axis you want.