Good Lord, EMACS is still being actively maintained. Who knew?
Back in the day, I could hack my way to a couple of useful macros: reformatting tricks, and I seem to remember making a VT100 act like a Prime Computer block-mode terminal, something like that. But I never drained the cup of Kool-Aid, like using the editor to run the COBOL compiler and my regression test suite.
(Link via Compiler.)