4. Sanity Effects - Eternal Darkness: Sanity's Requiem
2002 cult classic horror game Eternal Darkness: Sanity's Requiem may have been a commercial dud upon release, but it was widely lauded by critics for its groundbreaking "Sanity Effects" mechanic.
If the player character's sanity slipped sufficiently by coming into regular contact with enemies, they would be presented with visual and auditory hallucinations intended to trick them.
These hallucinations were incredibly advanced and convincing for their era, ranging from pretending to adjust the volume on your TV, reset the console, or delete your game save, to far more disturbing tricks like faking the protagonist's death.
Nintendo filed a patent for the sanity effects way back in February 2000, almost two-and-a-half years before the game's release, and though it was set to expire recently, the patent was renewed at the last minute.
While there have been games that have implemented similar ideas, such as Amnesia: The Dark Descent, they generally employ enough variation on the precise mechanics of the sanity meter to avoid invoking the ire of Nintendo's legal team.