Along the lines of one of the last post's items: Mimic.
Mimic converts input text into, well, an identical output. Visually identical output, that is. In reality, ASCII characters get replaced by very similar but non-equivalent Unicode characters, posing a challenge if that text needs to be parsed by anything other than a human.
It could have some use as a testing routine for code which absolutely has to tolerate Unicode strings as input. I mean, don't use this for anything else, even as a prank. Some lines just shouldn't be crossed.
I found Mimic through esoteric.codes, a wonderful project documenting the stranger and often more philosophical side of coding, including esoteric programming languages (esolangs).
See also: these Unicode toys.