Mirrodin: Chalice of the Void
Chalice of the Void enters the battlefield with X charge counters on it.
Whenever a player casts a spell with converted mana cost equal to the number of charge counters on Chalice of the Void, counter that spell.
This artifact can win a game all by itself. Ok, maybe not literally, but a Chalice set to 1 or 2 can shut down entire decks. In older formats that focus on efficiency, Chalice becomes even more brutal. It's restricted in Vintage, which says a lot.