I had intended to do up a post which was entirely horror webcomics, but these last 2 weeks have gotten away from me so I haven't gotten around to it.
Luckily someone at io9.com did, and their list pretty much has everything I was going to list and a good few more, so rather than present a half-formed thing check out io9.com: http://io9.com/looking-for-a-spooky-read-check-out-these-horror-webco-1652126842
One of my very favourites on this list is 'His Face All Read', a creepy short story whose style just oozes atmosphere. I mean, just look at it:
I'd be lying if I said that my original plan to draw up a list of horror webcomics wasn't just a means to share this comic.
So, yes, read it and enjoy.
I'll also draw special attention to the pants-sh*ttingly scary: The Bongcheon-Dong Ghost. I throw around the phrase "pre-sh*t your pants to get a head-start on the surprise of it later" but this comic pretty much defines that warning.
Everything after those two is a bonus.