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.
GreaseKit - GreaseKit is a SIMBL plugin, that adds user scripting to Safari, Mailplane, Diet Pibb.
Userscripts - An open-source userscript editor for Safari.
Man in the Middle - Modify requests, inject JavaScript and CSS into pages.