Even More Batteries Included with Emacs

Emacs features have a discoverability problem, and we're chipping away at it one demo at a time. The years since I wrote the last one of these have yielded more surprising and useful finds, so it's time again for a "batteries included" report.
Note This is the third in a series of articles highlighting useful but lesser-known features included in Emacs.
Parts 1 & 2:
Batteries included with Emacs More batteries included with emacs "Lesser-known" is a subjective judgment.