There are a lot of very useful userscripts on the Internet, these are just a few of the most useful ones.
No YouTube Volume Normalization.
https://gist.github.com/abec2304/2782f4fc47f9d010dfaab00f25e69c8a.
This userscript will perform volume normalization on Youtube videos, this will help out when you are using Youtube for Vevo.
Web Comic reader.
https://github.com/ameboide/webcomic_reader/blob/master/webcomic_reader.user.js.
Pretty good for reading webcomics on a large number of different pages like sadpanda.
A very nice screenshot script to capture native resolution stills from youtube videos with meaningful filenames with Ctrl+F11.
https://raw.githubusercontent.com/microsounds/atelier/master/Userscripts/youtube_screenshot.user.js.
Restor the old Arch Linux WIKI layout.
https://git.coom.tech/g/userscripts/raw/branch/master/arch-wiki-old-layout.user.js.
Play Youtube videos on the website with the MPV media player.
https://greasyfork.org/en/scripts/416271-play-with-mpv.
Automatically forwards country-blocked YouTube videos to eachvideo.com and unblocks the video.
https://greasyfork.org/en/scripts/9062-youtube-unblocker.
How To Write A Userscript: https://simply-how.com/enhance-and-fine-tune-any-web-page-the-complete-user-scripts-guide.