// Modify the default loop, include custom post types
global $wp_query;
$args = array_merge( $wp_query->query, array( 'post_type' => 'any' ) );
query_posts( $args );

// The beginning of the loop looks like this:
while ( have_posts() ) : the_post();

$args = array_merge( $wp_query->query, array( 'post_type' => array('post','news','essays') ) );

add_action( 'pre_get_posts', 'add_my_custom_post_type' );
/**
 * @param WP_Query $query
 * @return WP_Query
 */
function add_my_custom_post_type( $query ) {
    if ($query->is_main_query())
        $query->set( 'post_type', array( 'post', 'page', 'any_custom_type' ) );
    return $query;
}

add_action('pre_get_posts', 'add_my_custom_post_type');

/**
 * @param WP_Query $query
 * @return WP_Query
 */
function add_my_custom_post_type($query) {
    if(
        empty($query->query['post_type'])
        or $query->query['post_type'] === 'post'
    ){
        $query->set('post_type', array('post', 'my_custom_type'));
    }
}

and !is_admin()

add_action('pre_get_posts', 'add_my_custom_post_type');

/**
 * @param WP_Query $query
 * @return WP_Query
 */
function add_my_custom_post_type($query) {
    if(
        empty($query->query['post_type'])
        or $query->query['post_type'] === 'post'
        and !is_admin()
    ){
        $query->set('post_type', 'any');
    }
}