Show the Kitchen Sink by Default

^{Copying @nus’ answer as an answer to help newcomers.}

If you would like to show that second row of options by default, there’s an easy way. Simply put the following into your theme’s functions.php file:

function unhide_kitchensink( $args )
{
   $args['wordpress_adv_hidden'] = false;
   return $args;
}

add_filter( 'tiny_mce_before_init', 'unhide_kitchensink' );