An old scare story on Minecraft talk's about where a strange person with the default player skin and white eyes with no pupils appeared to a person, No one knows what he is but in that randomly generated map, Many things seemed off, Man made, etc, 2x2 tunnels going straight inside mountains, Neat little sand pyramids in the ocean, square plateaus, to neat and square for the random map generator to ever create. When the person who encountered him created a topic on the forums about it, It was soon deleted, He recreated it, but it was deleted again, Minutes later he recieved a email from a player named Herobrine which read "Stop". When the person tried to visit his Forums profile, It only lead him to a 404 error page.
Though this can be easily hoaxed, As to simply repainting an iron door model or a zombie model.
Notch has said that in the far future in a Seeecret Friday Update, That Herobrine will appear.
If Herobrine DOES appear in game, What features would you like him to have? What does he do, And what is the effect of it.
I would like him to just be what he is but appear randomly sometimes and scare the shit out of people, and he can appear behind you. And only some worlds can have him, Ones that do are squarish, and look more man made then randomly generated.