Kenko Tokina has a variable ND (neutral density) filter that you can adjust from ND2.5 all the way up to ND1000 – that’s stopping down to as much as 10 exposure values! You only have to turn its outer ring, much like turning it with a circular polarizing filter, to achieve the effect you want.
ND filters are very much like sunglasses for your camera. It darkens the scene to allow you to use a much slower shutter speed to blur, for instance, the flow of water from a waterfall to make it look silky soft and smooth. Or, when shooting a movie with a DSLR, you can open up aperture to blur the background even under a bright sun.
[Site: Kenko Tokina (Japanese)]