🐧 What are some cool little built-in gimmicks in linux, like for example /usr/share/cracklib/ or /dev/random?

I find it really cool that linux has many built-in features for development, coding, and just playing around. For example:

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/dev/random

Random-byte generator. Example use:

dd if=/dev/urandom of=~/urandom_test count=4 bs=1024

and you have a 4K file filled with random bytes.

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/usr/share/cracklib/

Just a collection of ~470,000 english words common passwords.

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yes

Continuously outputs the character y\n, for whatever reason.

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What other cool gimmicks are there?