This is timely for me. I have been known to use a neutral density (ND) filter occasionally, usually to lengthen my shutter speed to allow a longer expsosure, as was done by Sam M. in the image above. However, I carry a 3-stop ND filter, which allows 8 times the normal shutter opening time.
(Click Here) to see Sam M.'s post on mu-43.com
ND filters basically block a substantial portion of the incoming light, in a color-balanced way, allowing you to increase exposure in bright situations (either by lengthening the shutter opening time, or by opening up your f-stop for shallow depth of field).
In my Leica Akademie class on the M9 recently, our instructor, Justin Stailey, talked about 10-stop NDs. I had trouble imaging where I'd need that much of an effect. Sam M.'s image above answers the question well. Let's say you're shooting in broad daylight at iso200 - using the sunny 16 rule, you would want to shoot at about 1/200 second at f16. What would a 10-stop ND filter do for you? Well, if you wanted to make a long exposure, you could shoot at f16 for four seconds!
The shot above was made at 20 seconds exposure. That's what makes the clouds look so cool.
By the way, this effect would be very hard (impossible) to replicate in post-processing. The same is true if you use your ND for decreased depth of field.