Mess around with your friends, or come to their rescue - Someone Else's allows you to take ownership over certain items that other players have.
Stuff that works:
Someone Else's Ender Pearl - Craft with 4 pieces of paper and an Ender Pearl. Shift+Right-Click the target player and you have "their" Ender Pearl. Throw the pearl to watch them land where you threw them. You can also load an "owned" ender pearl into a dispenser to launch an ender pearl with redstone.
Stuff to come:
Someone Else's Armor and Tools - Damage is diverted from the player's armor to the target's - So is mending
The Someone Else's Team
