You can now use the app instance to store objects or parameters:

$app = new Application();
$app['twig'] = new \Twig_Environment(...);
$app['debug'] = true;

$app->get('/', function() use ($app) {
    return $app['twig']->load('index.html')->render(array());
});

For sercices, you can even make them lazy loaded and/or shared:

$app['twig'] = $app->asShared(function ()
{
    return \Twig_Environment(...);
});