In addition to moving water, what other motion blur applications can be achieved with an ND filter?
More or less anything that moves in relation to the camera.
One can make people moving through a scene disappear completely using enough density to require an exposure time of several minutes or more.
Imagine walking on a train while your assistant drives on a road parallel to the tracks. It keeps the camera pointing to the same point of the engine as the landscape passes. I have seen many examples of such "stimulation" shots.
You could even use an ND filter with a totally static scene and move the camera relative to the scene during a longer exposure to create a blur with a specific shape created by the camera's motion path.
Blur moving objects is only half of the equation that is obtained when using ND filters.
Instead of allowing a longer exposure time with the same aperture, ND filters can also allow wider openings with the same exposure times. Therefore, a very low depth of field can be used with large aperture lenses in bright environments.
Broader openings reduce depth of field. An ND filter allows wider apertures and more superficial DoF, even if the exposure time of the camera is limited by the flash sync speed or even the shortest exposure time of the camera.
For example, if you are outside on a bright and sunny day, but you want to use a very wide opening and fill flash. Your flash only works with an exposure time of 1/200 seconds or more. Your meter says you need to use a shutter time of 1/8000 to use f / 2. When using a 6-stop ND filter, you can use 1/125 seconds instead of 1/8000. (You'll also need a LOT more flash power!)