gistfile1.js

// ==UserScript==
// @name           someLink4Amazon
// @namespace      d.hatena.ne.jp/sugarbabe335
// @description    someLink4Amazon
// @include        http://*.amazon.*
// ==/UserScript==

var asin = document.getElementById("ASIN").value;
var td = document.evaluate('//form[@id="handleBuy"]/table[last()]/tbody/tr[1]/td[1]', document, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null).snapshotItem(0);

var links = {
"hatena" : "http://d.hatena.ne.jp/asin/{ASIN}",
"読書メーター" : "http://book.akahoshitakuya.com/b/{ASIN}",
"amazon" : "http://amazon.co.jp/dp/{ASIN}"
};

for ( var key in links ) {
  var div = document.createElement('div');
  div.innerHTML = '<a href="'+links[key].replace(/{ASIN}/, asin)+'">'+key+'</a>';
  td.appendChild(div);
}