Stop iTunes and Mac App Store from launching in Google Chrome while still maintaining the ability to manually launch with the button click. Modified the user script at http://userscripts.org/scripts/show/71794.

stopitunes.user.js

// ==UserScript==
// @name           Stop iTunes and Mac App Store Auto-Launch
// @version        0.1.1
// @description    Stop iTunes and the Mac App Store from automatically launching.
// @match          *://itunes.apple.com/*
// @match          *://phobos.apple.com/*
// @run-at         document-end
// @grant          none
// ==/UserScript==

// Version 0.1 seemed to stop working reliably with the stable version of Chrome 29. I had this leftover code
// below I found online (I changed the function name), but I forget where. If you wrote this or know where
// it's from I will be happy to attribute it.

function StopAutoLaunch () {
    window.onload = function () {};
}

addJS_Node (null, null, StopAutoLaunch);

//-- This is a standard-ish utility function:
function addJS_Node (text, s_URL, funcToRun, runOnLoad) {
    var D = document;
    var scriptNode = D.createElement ('script');
    
    if (runOnLoad) {
        scriptNode.addEventListener ("load", runOnLoad, false);
    }
    
    scriptNode.type = "text/javascript";
    if (text) scriptNode.textContent = text;
    if (s_URL) scriptNode.src = s_URL;
    if (funcToRun) scriptNode.textContent = '(' + funcToRun.toString() + ')()';

    var targ = D.getElementsByTagName ('head')[0] || D.body || D.documentElement;
    targ.appendChild (scriptNode);
}