I have two related suggestions, both are primarily useful in multiplayer, but both also have singpleplayer use.
Bloodhounds
Bloodhounds are dogs that can be used to sniff out players and villagers. To get a bloodhound, right-click a dog with an enderpearl. Bloodhounds can be right-clicked and they will sit, but it will open a menu to select what you want the bloodhound to track. You can type in any player's name and if (s)he's been on the server the dog will track him/her. If the player is gone the dog will track to his/her location when logging off. The bloodhound has the same stats as a normal dog.
Assassins
Assassins are black-robed testificates who can be hired to attack other players. Villagers can rarely spawn as them and they have 2 purposes.
If a village hates a player enough that it's golems attack him/her, then they will send assassins as well, though this only happens once per village.
If you trade with an assassin, you can give him 32 iron, 16 gold and 8 emeralds to assassinate a player, then select the player from the list of online players. Once paid he will disappear in smoke particles. Then 4-6 hostile assassins will spawn near the enemy player in light level 8 or below, and the assassins will try to kill the player. They assassins have the same amount of health as the player, and do 5-6 damage. They usually have iron helmets and sometimes an iron chestplate. One assassin will spawn with a diamond helmet and full iron armor, with a diamond sword. This assassin will have a piece of paper, named the name of the person who hired the assassins.
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This is way too OP, and can be used in PvP to find where any player is anywhere.
Edit: On second thought, I doubt this is a serious idea.