BikeShop - simple AJAX template manager (with caching!) for Handlebars.js. http://jsfiddle.net/jakebellacera/89XER/

bike-shop.js

// BikeShop
//
// Simple asynchronous template management for Handlebars.js.
// 
// By Jake Bellacera (http://jakebellacera.com)
//
// Usage:
//  1. Include Handlebars in your project.
//  2. Include BikeShop in you project.
//  4. Initialize a new BikeShop instance with the path to the templates dir as
//     an argument (example below).
// 
// Example:
//   templates/hello_world.handlebars
//   --------------------------------
//     Hello, {{ name }}!
//   
//   script.js
//   ---------
//      var shop = new BikeShop('/path/to/templates/dir'), // no trailing slash
//          element = document.getElementById('foo');
//     shop.render('hello_world', function (template) {
//       element.innerHTML = template({ name: 'World' });
//     });
//   
//   index.html
//   ----------
//     <div id="foo">Hello, World!</div>
var BikeShop = function(directory) {
  this.templatesDirectory = directory;

  return {
    cache: {},
    
    // Renders a template. If the template is not found within the cache,
    // fetch it asynchronously and render the callback when it has loaded.
    // 
    // Arguments:
    //   name     - (string) the name of the template.
    //   callback - (function) a callback function that will be fired after the
    //              template has been successfully fetched via AJAX. The
    //              callback function will be passed the raw Handlebars template
    //              as an argument.
    // 
    // Returns nothing.
    render: function(name, callback) {
      if (this.isCached(name)) {
        callback(this.cache[name]);
      } else {
        this.fetch(name, callback);
      }
    },
    
    // Fetches a template from a URL via GET, store the template in the cache
    // and then fires the callback function. If the template was previously
    // saved in the cache, replace the template with the new one.
    // 
    // Arguments:
    //   name     - (string) the name of the template.
    //   callback - (function) a callback function that will be fired after the
    //              template has been successfully fetched via AJAX. The
    //              callback function will be passed the raw Handlebars template
    //              as an argument.
    // 
    // Returns nothing.
    fetch: function(name, callback) {
      var shop = this,
          request = new XMLHttpRequest;

      request.open('GET', shop.pathFor(name), true);
      request.onload = function () {
        if (request.status >= 200 && request.status < 400) {
          shop.store(name, request.responseText);

          if (typeof callback === 'function') {
            callback(shop.cache[name]);
          }
        }
      };
    },

    // Checks if a template exists in the cache.
    // 
    // Arguments:
    //   name - (string) the name of the template in the cache
    // 
    // Returns true if it exists, false if it does not exist.
    isCached: function(name) {
      return this.cache[name] != null;
    },

    // Stores a raw Handlebars template into the cache. This can be useful if
    // you need to store a template manually.
    // 
    // Arguments:
    //   name         - (string) the name of the template.
    //   raw_template - (string) the string handlebars template.
    // 
    // Returns the raw Handlebars template.
    store: function(name, raw_template) {
      return this.cache[name] = Handlebars.compile(raw_template);
    },

    // Fetches a path for the handlebars template.
    // 
    // Arguments:
    //   name - (string) name of the template in the cache.
    // 
    // Returns the url for the template file.
    pathFor: function(name) {
      return this.templatesDirectory + "/" + name + ".handlebars";
    }
  };
};

But I ride my bike with no handlebars..