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.
Launchlet - Customize the web with JavaScript or CSS