combo.js

$$('select.combo').each(function(elm){
  elm.options[elm.options.length] = new Option('New...','New...');
  elm.observe('change',function(evt){
    if($F(this) == 'New...'){
      var text_input = new Element('input',{type:'text',size:'40',name:this.readAttribute('name')});
      this.insert({after:text_input});
      text_input.focus();
      elm.options.selectedIndex = 0;
      elm.hide();
      text_input.observe('blur',function(evt){
        if($F(text_input) == ''){
          elm.show();
          text_input.stopObserving();
          text_input.remove();
        }
      });
    }
  });
});