SuperMagnete documents a clever method for removing dents from brass instruments using powerful magnets. You insert a steel ball (smaller than the dent) in the instrument, and then use a padded magnet on the outside to "rub out" the dents. A more elaborate method uses a "reverse hammer" that works on harder surfaces.
To remove the dents on the hard parts of the instrument, Roberto developed a "reverse hammer". It is made of a non-magnetic rod (e.g. copper) with a movable heavy metal block on it's axis (e.g. the head of a normal hammer), a magnet on one end of the rod and a stopper on the other end (see drawing). When the hammerhead hits the stopper, a part of the resulting energy is transported through the magnet to the steel ball within the instrument, which "hammers" the dent from the inside.
(via Red Ferret)