Neutral density filters have been around forever, and depending on the manufacturer, are available in full-stop, half-stop, third-stop and even tenth-stop increments.  Depending on your needs, ND filters can be stacked together to create any number of density levels, based on how wide you want to keep your lens aperture or how slow you want to drag out your shutter speeds, without affecting the color qualities of the scene. ND filters also come into play when shooting film and video as a means of maintaining the critical exposure parameters inherent to cinema and video capture.