Neutral density filters | Can be used for good and evil

Neutral density filters reduce the amount of light entering the lens without altering it in any other way. It sounds an odd sort of thing to do, but ND filters have two important uses: (a) cutting the amount of light in daylight to allow slow shutter speeds; (b) darkening bright skies (graduated ND filters).

Graduated ND filters can be positioned carefully over the lens to darken the sky without affecting the rest of the scene (see the example above). They come in various strengths, usually expressed as a number. A 2x ND filter darkens the image by one stop, a 4x ND filter darkens by  two stops and an 8x ND filter darkens by three stops. It does take a bit of experience to gauge what strength of filter you need in any given situation.

Graduated filters slide up and down in the filter holder so that you can get the position exactly right. The filter holders have extra slots which allow the use of two or even three filters in combination.

You can also find 'neutral density' filters in some compact cameras. Here, they're used to simulate the effect of a smaller lens aperture (presumably it's cheaper than actually including a lens aperture mechanism in the lens). It does the job, adjusting the exposure just like a real lens aperture adjustment, but it will not produce the varying depth of field effects you might be expecting from aperture adjustments.