I discovered Creepypasta a few months ago and fell in love with the horror microfiction. I've always been a fan of things being short and sweet, which most of the stories on that website are. Although most of the stories were collected years ago off of 4chan's /x/ board, it's now powered by user-submitted content.
A lot of it is very amateur, but will most things creatively driven there are plenty of diamonds in the rough. Below is a list of some of the ones I've found that particularly sent chills down my spine:
Dust by Cameron Suey (aka Joseph K)
All the creepypasta I've read by Suey has been incredible, if not artistically inspiring. I remember reading this one a long time ago and loving it. I always like subtlety, and the way the narrator's character is woven through the story is amazing, and the ending is amazing.
Seventeen by Dave Taylor
This is a long one, but it really showed me where to start when I want to paint a literary picture. The pacing might be slow for some, but I find it added to the charm. It's scenes all had an eerie weightiness to them, much like the kind of dying world the narrator seems to be in.
In The Land of Black and White by Steven Harris
Death comes for a dying little girl, but not in the way most stories on that site go. This one, again, was amazing in it's delicacy with it's subject. Things never tip into terrifying, but the tension just builds up and up to it's eventual conclusion in an almost romantic fashion.
Hell At High Altitude by Brad Grandrino
The devotion and accuracy of the spelunking English explorer is to be admired, but most of all I loved thinking about how the author sat down and thought up this story. It's ending is like "The Sixth Sense", you really don't see it coming until it hits. But then, it changes how you approach every story in the future with a possible surprise ending.
I Am The Apocalypse by Lichtjunger
I have to admit, there is a very specific aspect of this story that I hated (or a specific choice in character, I should say). That aside, this story perfectly straddled the line of dissociative madness. The narrator's internal dialogue is so disarming, his perspective so rational. But then you get those twinges of madness, those moments of terror where something should be completely wrong but is written so nonchalantly. I don't normally like reading zombie stories, but this was good.