Melamine foam

Magic Eraser is a cleaning product comprised entirely of Melamine foam, which, because of its microporous properties, actually removes otherwise "uncleanable" external markings from relatively smooth surfaces. For example, it can remove crayon, magic marker, and grease from painted walls, finished wood, and car tires.

While this does appear miraculous, it's simply a property of the kind of foam used. The open cell foam is not only microporous, but its subtance, melamine, is also extremely hard, which means that it is actually working like sandpaper, but on a very tiny scale, getting into tiny grooves and pits in the subject being cleaned. Its structure ends up being more like a maze of fibreglass strands than like the array of bubbles of, for example, styrofoam.

The substance simply needs to be wetted to work properly, and it does break down rather rapidly, so that a given block of foam generally is seen to last only a single intensive scrubbing session, though it can be used repeatedly for much smaller marks.