To set the panning axis to plumb, most people just adjust the tripod legs so the platform is level, but for more precision there are leveling heads that go between the tripod and the head for making fine adjustments. Dan Fromm mentions the Bogen/Manfrotto leveling head, and I have a smaller Acratech leveling head. There's also one called the Levelhead, and some recent ballhead designs from Arca-Swiss and RRS put the panning platform between the ball and the camera instead of putting it under the ball, so if you level the camera, you can pan level.

The recent popularity of pano stitching has created demand for new solutions to the problem of level panning.