The tradition of hanging horseshoes above your front door is to ward off the devil himself.
Legend has it, that the devil, infuriated with the constant ringing of the hammer against the anvil, climbed up to earth to see where this terrible noise was coming from.
Arriving at the farrier's forge, he became curious about the horseshoes that the farrier was applying to the horse, and demanded that the farrier shod the devil's own hooved feet.
The farrier, recognising that this insistent and rather rude new customer was in fact the devil, nailed the shoes on still hot from the fire.
Soon, the heat from the shoes began to make the devil somewhat uncomfortable, but as the horse stood there calmly, the devil, not wanting to be upstaged by a horse, gritted his teeth while the farrier clenched up the nails. The job complete, the devil left the forge to test his new shoes on the cobbled street.
Soon the heat became so unbearable, the devil tore off the shoes and cast them as far as he could, vowing never to go near another horseshoe again.
The debate continues as to which way up it goes, but for many years, the simple horseshoe has been displayed above many doors, and from what I hear, the devil hasn't been seen in any of them.