S is for Slender Man
Slender Man resembles an extremely (inhumanly) tall and thin 'man' who appears to be wearing a suit. Sometimes he appears to have an excessive number of arms, although this varies from sighting to sighting. According to legend (more on this later) he generally targets children, and makes them disappear. He's not opposed to attacking adults and will sometimes leaving their corpses hanging in trees, devoid of internal organs.
He is also, completely, a product of the internet. It's possible to read the very first post Slender Man ever appeared in, and the thread through which his legend grew. MarbleHornets (which I recommend if you really like getting the pants scared off of you) was born in the same thread.
But setting aside all the terrifying fiction, what makes Slender Man really fascinating is that since his inception there are people who have been claiming to see him. Some people believe that this is just people scaring themselves silly, which is possible. However, there are some people who believe that Slender Man may have been willed into being. That enough fear and work and creative energy has gone into him that he actually now exists.
There is definitely something to the Slender Man, he taps into some primal fear that effects nearly everyone. The imagery and stories that go around about him are creepy and dark and scary, and they rarely end well. There's something impressive about an image so powerful that even though he's provably invented, people still claims he's real, and to have seen him.
Photos are courtesy of Victor Surge, who can be found on his Deviant Art with many more Slender Man photos. He may or may not be working on a book about Slender Man.