You need to turn off the layout renderer and the view renderer:

public function busyAction()
{
    //
    // turn off the view helpers
    //
    $this->_helper->layout->disableLayout();
    $this->_helper->viewRenderer->setNoRender();
    //
    //
    //
    $tmp = array( 'test'=>'value' );
    echo Zend_Json::encode($tmp);
}

Then use Zend_Json to encode your values and output them.

For the client side I suggest you grab jQuery or prototype to make things easier.