Hearthpwn

"HEY, I've got one of those too! One of us is going to need to change." 

Most of the time, this is just hard removal, however, play this againest a card you also have a copy of in your deck and are rewarded.

What this means is that both players essentially automatically will just end their next turn (can stack), which in most cases will result in both players drawing 1 card and gaining 1 mana crystal.  However, other effects are useful such as end of turn effects AND start of turn effects.  Effects like Freeze also are affected as it will essentially unfreeze all units (as the turn they were frozen has passed).

Notable combos could include Doomsayer or Jan'alai, the Dragonhawk (Or Wild: Emperor Thaurissan

Uh oh, nearly forgot to put in my submission. Here ya go!