There's also a wobbly drill or oscillating drill that will do the same, but it best suited to a milling machine or stiff drill press. Not a hand tool, but then, neither is a mortising bit....
When drilling holes in thin sheet, like aluminum aircraft skins, a regular 2-flute drill wants to make a triangular hole. The video shows how this can be a "feature" instead of a defect...
The Divot