Known as The Tallman, Slenderman, and The Hatman, this urban legend is reported to be very tall, very slender, and wearing a suit and having a pale gaunt face. Sounds kind of like the Tall Man in "Phantasm."
The story goes that his arms can twist and bend and reach long distances, scratching on windows with talon-like fingers. Like shadow people, he's often found in wooded areas, hiding among trees. He is also seen sneaking down hallways and showing up in homes.
The explanation for where he comes from involves a theory that he was tortured. He would stand over your bed and ask you a question. If you answered him correctly, he would just break your limbs, but if you answered wrong, he would pull your heart out through your throat. This gives the appearance of having aspects adopted from Germanic and Romanian stories that were used as cautionary tales for citizens wandering the lonely woodland paths.
Interestingly, after its popularization on the Internet, many people jumped on the bandwagon claiming extraordinary encounters and the fear and excitement rose. There is something elemental in this legend that strikes a note for people and keeps it a growing part of paranormal beliefs.
I have to wonder about the Tallman and his relationship to the hat-wearing shadow person many report. Even though the majority of shadow people are small, perhaps 3-4 feet tall, there are also tall, slender, hat-wearing ones reported.
Until I hear some compelling stories of the Tall Man, I'm keeping it in the category of cautionary tale/urban legend.