Over at the Emacs subreddit, Phovox—a 30-year Emacs user—asks what your favorite Emacs utilities are. He’s started using Doom Emacs and has discovered, through Doom, utilities that he didn’t know about. So he’s wondering what else he may have missed and is asking the community what they recommend.
Some of the answers were general things like the ability to use Emacs to change virtually any part of Emacs to suit your preferences. Others were more on point and included things like Dired, Hippie-expand, the classical version control system, Ediff, Magit, Calc, Org, Projectile, LSP, and Replace-regexp.
Some of the other commenters mentioned third-party packages such as Hyperbole and Everything.
The best advice, I think, came from Wumpitz. That is to spend some time with the Emacs manual. It’s too much to try to read it straight through, of course, but you can decide to read one chapter every day or week. It is, says Wumpitz, amazing how much you don’t know about some of the basic utilities.
All the action is in the comments so take a look to see if there’s some utility you should be using but aren’t.