throbbing.js

$(document)
  .ajaxSend(function (event, xhr, aOpts) {

    $('<div/>', {
      id:   'loading-indicator',
      text: 'Working...',
      css: {
        position: 'fixed',
        top: '0',
        left: '0'
      }
    }).prependTo('body');    
  })
  .ajaxComplete(function() {
    $('#loading-indicator').remove();
  });