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FireMonkey - Super Lightweight User Script and Style Manager
Violentmonkey - Violentmonkey is a userscript manager to support running userscripts in web pages.
Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Greasy Fork - A site for user scripts.
Tampermonkey - Greasemonkey compatible script manager.
Userscripts - An open-source userscript editor for Safari.