This isn't yet public on the main site, but as usual, it's being posted here first:
Tutorial: Neutral Density Filters
It mentions some specific examples, and discusses all the common uses for these filters. There's also a section on using image averaging (aka "stacking") to effectively create a digital ND filter, along with its potential complications.
This article rounds out the series on camera lens filters. Polarizing, GND and ND filters are all now receive separate discussions.
As usual, please let me know if you feel anything is unclear, if you notice any typos, or just want to add something from your own experience.