ROT13
A Caesar cipher with a shift of 13 — its own inverse, and never intended as security.
Each letter moves 13 places along the 26-letter Latin alphabet. Because 13 is exactly half the alphabet, the operation is its own inverse: the same function both encrypts and decrypts, which makes it trivial to implement. That made it popular on Usenet from the early 1980s, where it was used to hide punchlines, spoilers and offensive posts from anyone who did not actively choose to read them.
As protection it is worthless, and that was never the point — it is a courtesy marker, not a lock. The term survives as a standing joke among cryptographers: "ROT26" for applying it twice, meaning no encryption at all, and the promise to protect sensitive data "with ROT13, twice, to be safe".