What were some slept on cards that ended up being extremely strong in competitive formats?

The main card I'm thinking of is \[\[Ledger Shredder\]\]. When it was revealed, people had the pre-order price around $5. Shortly after it was released, it saw play in several formats and its price rose to $25+ for a while. It became a card that people were taking in drafts solely for the money, even if they weren't playing Blue.

What are some other seemingly innocuous cards that surprised people by how prevalent they were in a certain format after they were released? They don't necessarily need to have climbed in price. I just want to know about which cards surprised people by how good they were in a certain format.