Tampermonkey: Amazon Smile

// ==UserScript==
// @name        Amazon Smile Redirector
// @namespace   http://danrossiter.org/
// @version     1.0
// @description Redirects all Amazon URLs to Amazon Smile subdomain. Also handles potential for infinite looping when smile subdomain doesn't exist.
// @match       http*://amazon.com/*
// @match       http*://www.amazon.com/*
// @grant       unsafeWindow
// @copyright   2014+, Dan Rossiter
// ==/UserScript==

var cont = true;
var query = window.location.search.substring(1);
var vars = query.split('&');
for (var i = 0; i < vars.length; i++) {
    var pair = vars[i].split('=');
    if (pair[0] === 'dr_smile') {
        cont = false;
        break;
    }
}

if (cont) {
    query = '?dr_smile' + (query ? '&' + query : '');
    window.location = window.location.protocol + '//smile.amazon.com/' + window.location.pathname + query;
}