Which command line tools do you use?

I’ve been watching Prag Dave’s Elixir course and I noticed he uses tree:

Tree is a recursive directory listing program that produces a depth indented listing of files. Color is supported ala dircolors if the LS_COLORS environment variable is set, output is to a tty, and the -C flag is used. With no arguments, tree lists the files in the current directory. When directory arguments are given, tree lists all the files and/or directories found in the given directories each in turn. Upon completion of listing all files/directories found, tree returns the total number of files and/or directories listed.

On Mac install with: brew install tree

EG:

$tree
.
├── README.md
├── lib
│   └── hangman.ex
├── mix.exs
└── mix.lock

You can use the options to ignore directories or files.

Any other cool command line tools you know of or use?