What cards are underestimated, and more useful than they appear?

What are some cards that seem weak, but we might just be using them wrong? Maybe some of their use cases just aren't obvious. I'm sure there are plenty of cards that get overlooked. I'd like to know what value I might be missing out on.

The one that comes to mind for me is Sin'Dorei, with people not knowing when to use it, and only seeing it as an anti-Automaton tech.

It's useful against reborn natural-Taunts, like Rylak and the Tiger, and the t1 Undead, basically whenever Taunt minions are protecting your opponent's critical minions.

It can clear 2 Taunts, 2 Divine Shields, and 2 Reborns if you're lucky, and even a Venomous. It can give you access to their lynchpin minions, so that you might even kill them when your second minion attacks.

I think Sin'Dorei deserves more credit.