Aside from moving water, what other motion blur applications can one achieve with an ND filter?

Pretty much anything that moves in relation to the camera.

One can make people moving through a scene totally disappear by using enough density to require an exposure time of several minutes or longer.

Imagine pacing a train while your assistant drives on a road parallel to the tracks. You keep the camera pointed at the same spot on the engine as the scenery flows by. I've seen many examples of such "pacing" shots.

One could even use an ND filter with a totally static scene and move the camera with respect to the scene during a longer exposure to create a blurry effect with a specific shape created by the path of the camera's motion.

Blurring moving objects is only half of the equation afforded by using ND filters.

Rather than allow a longer exposure time with the same aperture, ND filters can also allow wider apertures with the same exposure times. Thus one can use very shallow depth of field with wide aperture lenses in bright settings.

Wider apertures reduce the depth of field. An ND filter allows wider apertures and shallower DoF even if your camera's exposure time is constrained by flash sync speed or even the camera's shortest exposure time.

For example, if you're outside on a bright, sunny day but you want to use a very wide aperture and fill flash. Your flash only works with a 1/200 second or longer exposure time. Your meter says you need to use a shutter time of 1/8000 to use f/2. By using a 6 stop ND filter, you can use 1/125 second instead of 1/8000. (You'll also need a LOT more flash power!)