Creepiest Anime/Fictional Thing I've Seen:
Someone here mentioned Elfen Lied. I've seen a few episodes of that and it is a good choice for disturbing anime (seems more people whov'e watched it are more upset about the thing with the dog than about all the human murders... ) but I'm going to up the ante with something you've probably never heard of:
Requiem from the Darkness. It's a horror anime that's also known as The 100 Stories (but the English name was changed so as not to confuse it with a porno with a similar name). Requiem has some racy content but is anything but titilating: It's very dark. It follows a young author in Edo period Japan seeking out the origins of ghost stories. He ends up following a group of travelers and uncovers many "ghost" stories based on entirely human darkness (people covering up murders and such). Just for a taste: There's the story of a lord whose gluttony has turned him into a blob - he demands his servants procure him many different kinds of meat because he once had to eat a horse to survive a bad situation and never tasted something so wonderful as that horse's flesh. Let's just say that we find out at the end of that little story that his survival situation didn't have him eating a horse... It's a good series if you like horror, only thirteen episodes. I ought to re-watch it since I haven't in a while. It just is... intense...
Creepiest Online Thing I've Seen:
Exit Mundi - A collection of end of the world scenarios. Most of them are based in science and most of them can happen any day. (I actually enjoy reading through stuff on this site every now and again). If you want to write an apocalyptic story, this is the place to go for inspiration.
Creepiest Thing I've Seen in Real Life:
A horse get turned inside out by a car right in front of me. Saw that last summer. I told the story of it on my blog - look up June 2011 if you want to read about it.