What is your favorite urban legend?
As a child, the ones that frightened me the most were Mikey and the exploding Pop Rocks/Pepsi stomach (discredited, but it used to give me nightmares), and Bloody Mary (which I never tried.) The most intriguing aspect of the latter legend is that the Bloody Mary story didn't exist before the 80's, according to folklorists. As I was a child in the '80s, I knew of the legend.
Edit By the way, feel free to include non-deadly urban legends. All this talk of Bloody Marys, banshees, and murderous vampires is giving me the creeps. :-)