Hi,
I bought 2 neutral density gray filters some weeks ago. I would like to show you some results.
ND filters are used to take away light from the scene, to a certain amount.
There are two different types of ND filters on the market: fixed stop value filters, and variable filters. The latter work with two polarizing filters rotating over each other, and they have a stopped down value between 3 and 8 stops. The fixed value filters consist in a single sheet of shaded gray plastic or glass or whatever. The Polarizing filters version has the advantage that you can vary the number of stops of light blocked, they have the disadvantage that they create an interference pattern when used at the darkest setting (showing as a darker cross in your image, especially when used with wide angle lenses on a full frame body)
I have bought a 3 stops (also called 8x because it allows only 1/8th of the light to pass) and a 10 stops (also called 1000x) ND filter, for my 17-40 mm wide angle lens.

a 1000x ND filter blocks 999/1000 of the light, this makes it possible to use long exposure times in full daylight. The examples below are from 20-80 seconds for the exterior images, 400 seconds for the interior images. This makes all moving subjects fade away, to the point of becoming invisible if they do not halt during the exposure time. I have taken images of a crowded train station hall (Gent St.-Pieters) a very crowded shopping street (Gent Veldstraat), the E40 Highway on a very busy day, … without moving subjects visible.
One remark, the 1000x ND filter creates a lot of vignetting and a white balance shift towards red, I’m not sure if this is a typical ND filter thing or specific for the ones I got.
I also joined two images without the use of the ND filter, to make obvious what the effect is. Click for full size!