Neutral density filters can be used to ensure that you can use large aperture even in daylight, for example to ensure blurred background in portrait photos. A variable neutral density filter may be suitable for this purpose.
A neutral density filter can also be used to ensure long shutter speeds to give the special soft effects of water and clouds. For this purpose the dark filters that can reduce the light by 10 stops are suitable.

There are variable neutral density filters called ND 2-400, which reduces the light with 1-9 stops.
- And there are solid neutral density filters called ND x2, ND x4, x8 .... ND x1000, which reduces light respectively 1,2,3 and 10 stops.

Below is a conversion table can be used to find the correct shutter speed when using ND filters.
If You have to use 1/60 second to achieve proper lighting, you must by using NDx1000, that reduce light with 10 stops, use a shutter speed of 15 seconds.
By clicking on the table, You will get a table, that can be printed.

The first ND-filter I bought was a variable Hama ND 2-400, but it did not work with wide angle and the greatest reduction of light with 8 or 9 steps. Either were two of the corners very dark or very light. Therefore, I returned the filter.
Instead, I bought a solid neutral density filter, B + W ND 3.0 1000X. My purpose was to play with slow shutter speeds.

With filter on there are a slight vignetting, but it is easy to remove in post-processing. On the web you can read that the filter should give warmer tones, but I think the colors looks fairly neutral. There have been no color correction of the image.

Using a 10 stop Nd-filter (Neutral Density filter)

The filter can't be used without a stable tripod. The filter is so dark that neither the camera rangefinder or light meter works. You will have to focus and compose the image without the filter is attached. Find the right combination of aperture and shutter speed without the filter attached.
Switch the lens to manual focus. Convert the shutter speed using the table further up the page. Then attach the filter and take the picture.

ND-filters that can be recommended

By searching the web I have found brands, that are relevant to consider if you want to buy an ND-filter.

If you want to work with very long exposure, a 10-stop ND-filter are recommended. Such as Tiffen 3.0 Neutral Density Filter, Hoya Pro ND 1000, Heliopan Neutral Density 3.0 and B+W ND 3.0 MRC 110m.

Are the purpose to use large apertures in daylight, a variable ND-filter are suitable. Recommended filters are Heliopan Variable ND filter, Eclipse ND Fader, HOYA Variable ND filter and a cheap Chinese filter, Haida PRO II Variable ND Filter.