Unmasked Identity
Very useful on heroes with a strong early game Hero Power or one that is one-time use only, like Reno Jackson.
For heroes which lock minions in your hand (like Thorim, Maiev or Faelin), the unlock condition is tied to the minion. That means you can use this spell to switch your hero power at any time and still get the minion later.
It is possible to discover the same Hero Power more than once, during one game.