gistfile1.txt

UI.registerHelper('eachIndex', function () {
  var withIndex = this.map(function(x, i) {
    return {data: x, index: i}
  });
  return UI.Each(
    function() {
      return withIndex;
    }, UI.block(null), UI.block(null));
});

Then you can use #eachIndex instead of #each and access the data and the index using dot notation:

<template name="myTemplate">
  <ul>
    {{#eachIndex myList}}
           <li>Item number {{this.index}} is {{this.data}}.</li>
    {{/eachIndex}}
  </ul>
</template>