For a simple example, Acme has no paragraph justifier, but it doesn't need one, since fmt exists. To use it, highlight a paragraph of text with the left mouse button, highlight a typed-in |fmt command with the middle mouse button, and release the middle mouse button to run the paragraph through |fmt and replace it in place.

With this I can leverage anything I can call from the command line, in any language, to be part of my editor. No need to rewrite tools in the program's specific extension language, whether Elisp or Smalltalk or Clojure.

As another example, since it's so easy to just run a shell command from Acme (highlight the command text and middle-click it), I'm constantly customizing pre-written shell commands to e.g. grep for lines in code (to the point of creating indexes on the fly for files), and then right-clicking on grep -n output to jump to a specific line in a file.