Note: After saving, you have to bypass your browser's cache to see the changes. Internet Explorer: press Ctrl-F5, Mozilla: hold down Shift while clicking Reload (or press Ctrl-Shift-R), Opera/Konqueror: press F5, Safari: hold down Shift + Alt while clicking Reload, Chrome: hold down Shift while clicking Reload.
if ( typeof $ !== 'undefined' && typeof $.ready === 'function' ) {
    addOnloadHook( $.ready );  // just a test...
}

if (wgNamespaceNumber == 6) addOnloadHook(function () {
    var url = "http://toolserver.org/~vyznev/cgi-bin/checkusage.pl?files=" + encodeURIComponent(wgTitle);
    mw.util.addPortletLink("p-cactions", url, "Q&D CheckUsage", "ca-qdcheckusage", "Vyznev's quick and dirty file usage checker");
});