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Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Tampermonkey - Greasemonkey compatible script manager.
Violentmonkey - Violentmonkey is a userscript manager to support running userscripts in web pages.
Userscripts - An open-source userscript editor for Safari.
Man in the Middle - Modify requests, inject JavaScript and CSS into pages.
Launchlet - Customize the web with JavaScript or CSS