@niklas m. - Leashing to Player

To obtain your UUID pair, you need NBTExplorer: http://www.minecraftforum.net/topic/840677-nbtexplorer-nbt-editor-for-windows-and-mac/

When you open up your world in the program, you can obtain your UUID pair by navigating to your player file. See here: http://i.imgur.com/PaKJykk.png

Once you've gotten a hold of your UUID, you can apply it to leashes. You MUST label the tag-type of the UUID tags, which are type "long" (thus labeled with an "L") For example:

/summon Creeper ~ ~1 ~ {Leashed:1,Leash:{UUIDLeast:<NUMBER>l,UUIDMost:<NUMBER>l}}

The UUIDLeast and UUIDMost must appear in the order that they appear on your player file. So if UUIDMost comes first, you have to label that first. Same goes with summoning a mob with their own UUID.

So using my own UUID:

/summon Creeper ~ ~1 ~ {Leashed:1,Leash:{UUIDLeast:-9192300185346116154l,UUIDMost:-5068141680530666168l}}

Note that this CANNOT be automated, so you cannot automatically apply leashes to players.