Which Linux command or utility is simple, powerful, and surprisingly unknown to many people or used less often?

This could be a command or a piece of software or an application.

For example I'm surprised to find that many people are unaware of Caddy, a very simple web server that can make setting up a reverse proxy incredibly easy.

Another example is fzf. Many people overlook this, a fast command-line fuzzy finder. It’s versatile for searching files, directories, or even shell history with minimal effort.