I don't scare easily. I laughed during The Exorcist and was almost entirely unphased during Paranormal Activity. But a YouTube series that's probably cheaper than dirt to make has me entirely freaked out, even though I know the entire thing is fake. It's Marble Hornets, and if you're familiar with Slender Man lore on the internet you know all about it. If not, don't turn to Google - just watch the introduction and Entry #1 and see what happens.
It's like a combination of H. P. Lovecraft, Signs, The Ring, and a good version of The Blair Witch Project. Really unnerving, scary, and a well-done mystery.