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Closest I can think of is Stat-O-Vision.
A viewer external to the game? What are a few examples of that?
Becky: Who are you? The Mysterious Stranger: An angel. Huck: What's your name? The Mysterious Stranger: Satan.
Like something from http://dwarffortresswiki.org/index.php/DF2014:Utilities
?
It's increasingly common for online web games and mobile games. Kantai Collection and Touken Ranbu have them, as do Granblue Fantasy and Fate/Grand Order. As an example, the first one has Kan Colle Viewer.
In some games where people do speedruns or other things they don't just use emulators; they also use such software to look at hidden stats like the speed of the player character, like what this guy did for Super Mario 64.
What is the trope for when software is made for a video game that doesn't actually modify anything within the game, but just displays stats and other things that the game doesn't normally show, making it easier to play? An external viewer of sorts.