Tampermonkey is the most popular userscript manager, with over 10 million users.
Tampermonkey is used to run so called userscripts (sometimes also called Greasemonkey scripts). Userscripts are little computer programs that for example add download buttons to YouTube pages, cleanup your Facebook timeline or help playing an online game.
https://chrome.google.com/webstore/detail/tampermonkey/dhdgffkkebhmkfjojejmpbldmpobfkfo
@ecksmc You just need to be careful to trust the person who wrote the script you are running. (I've used it for years as it is a popular way to get around the extension restrictions whenever they change).