Show all registered widgets in wp-admin

show_registered_widgets.php

function show_widget_classes() {
    global $wp_registered_widgets; 
    $widgets = array(); 
    if(is_array($wp_registered_widgets)){ 
        foreach($wp_registered_widgets as $widg){ 
            if(!empty($widg['callback'])){
                if(!empty($widg['callback'][0])){  
                    $class = get_class($widg['callback'][0]);
                    if(!array_key_exists($class, $widgets)){
                        $widgets[$class] = $widg['callback'][0]->name;
                    }
                }
            }
        }
    }
foreach($widgets as $widget_class  => $widget_title ){
        echo '<pre>'; print_r( $widget_class ); echo '</pre>';
    }
}
add_action( 'admin_notices', 'show_widget_classes' );