A very old skool editor which has survived to become a power-tool.
Philosophy of Emacs is a tiny core in C which manages a buffer of text, but includes a scripting language EmacsLisp out of which can be built almost any tool or functionality imaginable.
Much of Emacs's core functionality is built in EmacsLisp. And then others have added lot more.
I use it for ClojureLanguage with ParEdit
There's a long history of writes and re-writes, but the most common version today is the one that was largely written / launched by RichardStallman as a flagship Gnu project.