I have a zombie spawner set up to farm exp, and I also have a skeleton spawner set up to do the same.
Because I'm using them as exp farms (getting them down to half a heart and then punching them to death), I also occasionally get rare drops from them both.
Around about a quarter of the bows that drop come with a Power I enchant, and I've gotten two enchanted golden swords from Zombie Pigmen (regular kills), but I've never had an enchantment on any of the hundreds of swords/shovels/helmets that the Zombies have dropped.
Which items have a chance of being enchanted upon their drop?