Quick maintenance mode in WordPress. Only admin can browse the blog, other people get a maintenance message. Put this php file in /mu-plugins.php

maintenance.php

<?php
/**
 * Quick maintenance mode in WordPress, using a bit of custom css and html
 * 
 * Only admin can browse the blog, other people get a maintenance message
 * 
 * Put this file in /wp-content/mu-plugins folder
 */

function alo_maintenance_mode() {

	if ( ! current_user_can( 'administrator' ) ) {

$html = '
<style type="text/css">

html {
  /* E.g. force a custom background */
  /* background-color: #efefef !important; */
}

#error-page {
  /* Customize the message box: width, background... */
}

/* E.g. put the message on right */
#msg-alert {
	display: block;
	font: 20px verdana, arial, sans-serif;
	position: relative;
	text-align: right;
	line-height: 110%;
	color: #444444;
}

</style>

<!-- E.g. put here your logo, on left -->

<!-- Write your alert message -->
<span id="msg-alert"><strong>Maintenance Mode.</strong><br /><small>Please try back in a few hours.</small></span>
';
		wp_die( $html, 'Maintenance Mode | ' . get_bloginfo('name') );
	}
}
add_action('get_header', 'alo_maintenance_mode');