Outstanding Gameplay Mechanics

We all talk about the some of the best games we've played, but what about gameplay mechanics?

Great gameplay mechanics can come from the worst games to the greatest. Some of the best gameplay mechanics show up in the most forgettable titles, not to be revived until years later in another developer's game. They can be in the form of great/unique controls, inventory/item management, or just how you interact with the game-world in general.

Can you describe a gameplay mechanic that has stood-out to you? What about the game (good or bad) it came from?

Just Cause 2 - Tether/Parachuting Mechanic

Ratchet and Clank - The Weapon Wheel