If your WordPress deployment has multiple stages (ie local, staging, production), it can be helpful to have a visual reminder of which stage you are currently viewing in the browser. This adds a colorful box to the Admin Bar displaying the current environment, based on the value of WP_ENV defined in the wp-config file. Drop it into mu-plugins.

environment-flag.php

<?php
/**
 * Plugin Name: Environment Flag
 * Description: Show the current environment (ie. local, staging, production) in the Admin Bar.
 * Version: 1.0
 * Author: Corey McKrill
 * Author URI: http://coreymckrill.com/
 * License: GNU General Public License v2.0
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

if ( ! class_exists( 'CMM_Environment_Flag' ) ) {

  class CMM_Environment_Flag {

		function __construct() {

			add_action( 'admin_bar_menu', array( &$this, 'add_flag' ), 15 );
			add_action( 'wp_before_admin_bar_render', array( &$this, 'flag_style' ), 99 );

		}

		function add_flag() {

			global $wp_admin_bar;

			if ( ! is_user_logged_in() ) { return; }
			if ( ! is_super_admin() || ! is_admin_bar_showing() ) { return; }
			if ( ! defined( 'WP_ENV' ) || ! is_string( WP_ENV ) ) { return; }

			$args = array(
				'id' => 'environment-flag-' . WP_ENV,
				'title' => ucwords( WP_ENV ),
				'parent' => false
			);
			$wp_admin_bar->add_node( $args );

		}

		function flag_style() { ?>
			<style type="text/css">
				#wpadminbar ul li#wp-admin-bar-environment-flag-local { background: orangered; }
				#wpadminbar ul li#wp-admin-bar-environment-flag-staging { background: yellow; }
				#wpadminbar ul li#wp-admin-bar-environment-flag-production { background: green; }
			</style>
		<?php }

	} // end class

	new CMM_Environment_Flag();

} // end if