Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Amino Editor - Amino is a Chrome browser extension for customizing web page presentation with user CSS.
Violentmonkey - Violentmonkey is a userscript manager to support running userscripts in web pages.
Stylus - User Styles Manager - Stylus is a userstyles editor and manager based on the source code of Stylish version 1.5.2.
GreaseKit - GreaseKit is a SIMBL plugin, that adds user scripting to Safari, Mailplane, Diet Pibb.