GreaseKit is a SIMBL plugin, that adds Greasemonkey-like user scripting to Safari, Mailplane, Diet Pibb.app and all WebKit applications. Greasemonkey scripts are not 100% compatible with GreaseKit because Safari’s JavaScript implementation is not compatible with Firefox’s and GM_registerMenuCommand is not available in GreaseKit yet. Many scripts, however, will work in GreaseKit with little or no modification.