Emacs
A fully extensible editor — at its core a Lisp interpreter that happens to display text. GNU Emacs (Richard Stallman, 1985) is the dominant variant.
"A great operating system, lacking only a decent editor." Everything — from syntax highlighting to a mail client to Tetris — is written in Emacs Lisp and modifiable at runtime. Popular distributions: Doom Emacs, Spacemacs, vanilla + use-package.
Today: org-mode (planning, documentation, literate programming) is its unique strength. LSP support, Magit (famously the best git interface), tree-sitter integration. Competitors: Neovim, VS Code. Adoption has dropped but the cult lives on.