Hanging a horseshoe over a door is considered to be an omen of good luck. Hanging the horseshoe properly is key, if you hang it so that the two points are pointing down the luck may run out of the horseshoe. The origins of the horseshoe's luck derive from Catholic legends.
The legend involves Saint Dunstan, a former blacksmith, was asked one day by the devil to shoe his cloven hoof. As many Catholic's, or horror movie fanatics will know, Satan often has the legs of a goat, hooves and all. Dunstan decided to perform the favor for the devil but in doing caused the devil such great pain that the devil promised to never return to a house with a horseshoe hanging over the doorway.
Personally I'm always a little scared the horseshoe is going to fall off the doorway and hit me in the head, have you felt how much one of those suckers weigh?
Saint Dunstan lived from 909 to 988 AD, for more on him check this link out:
http://www.newadvent.org/cathen/05199a.htm
