I watch the Friday the 13th movies about once a year, as they are my '80s comfort movies, and I still enjoy them. The DVD and Blu-ray commentaries are also a great listen. The 80-hour (exaggeration intended)
Crystal Lake Memories documentary is a great walk down memory lane, as well.
There is also a
Nightmare on Elm Street documentary, by the same guys, that is a good watch. For general horror documentaries, the
In Search of Darkness series is pretty good, albeit there are some annoying talking heads in it. Here's a pretty
good list of horror documentaries if you have a hundred or so hours to kill. I seem to have gone on a horror documentary tangent...