Strionic Resonator

Triggered abilities are the most quietly powerful effect-class in Magic, and for years almost nothing could double them. Spells got Twincast and Fork; activated abilities got Rings of Brighthearth, this artifact's spiritual twin. But "when," "whenever," and "at" went untouched, which left an enormous surface of value (etb effects, attack triggers, end-step engines, combat damage) sitting unduplicated until this card put a wrench on it for two mana and a tap. The design holds together because the cost stacks: each copy is its own activation, so you pay the two and tap every time, and the Resonator produces no mana to feed itself. That ceiling keeps it an engine piece rather than a guaranteed kill, since there is no free loop hiding inside it. The choose-new-targets clause is what gives it reach beyond raw multiplication, redirecting a copied trigger somewhere the original could not go. It rewards the player who notices a trigger on the stack while the window is still open, because the copy resolves before the original. Narrow by intention and useless without something worth copying, it nonetheless opened a design axis that triggered-ability synergy decks had been circling for years.