http://www.slendergame.com/
It's so incredibly scary. check out the link on the website of the reaction videos of people playing it, these are people with their video game channels on youtube, a couple of them started crying they were so scared, it's hilarious (especially one guy who was talking sh!t when he first started playing it and then lost it when the scary part came).
Seriously, I thought I was going to have a heart attack when I was playing this, it really screws with your head. Game length is about 5-15 minutes. If you're a real masochist try playing this alone at night, you're guaranteed nightmares. I played it during the day with a movie playing next to me and I was still freaking out. I don't care if you're a gamer or not, this is something you gotta experience at least once.
For any who are interested. Here are a couple of very vague hints when playing the game because you get no instructions when you start except to collect the 8 pages, but that's part of the atmosphere of the game. These hints won't spoil much but I recommend playing the game through at least once before reading them so as to get the full impact (don't worry, you won't last long the first time hehehe)
Spoiler :
