Dial A Darkness With Promaster
The variable ND finds a lot of favour with the videographers who are using their DSLR cameras as it allows them to fade out os a shot by dialling the side of the filter. It could also be a good idea for the still worker who wants to darken scenes but doesn't want to have to carry a whole pouch full of filters.
The principle of the thing is simple - two polarising filters set in a revolving frame - as they rotate one cancels out the polarised light from the other and it all goes dark. As they are working in opposition you don't really get the classic polarising effect on water or glass
These Promaster ones are slim-line so you can avoid vignetting. The power of the filter ranges from ND3 to ND400. Look at the two images for the extremes of the range.