Uses CSS to minify the Wordpress admin bar into a button. Expands on hover.

minify-bar.php

<?php
/*
 * Plugin Name: Mini Admin Bar
 * Plugin URI: http://www.netyou.co.il/
 * Description: Makes the admin bar a small button on the left and expands on hover.
 * Version: 1.0
 * Author: NetYou
 * Author URI: http://www.netyou.co.il/
 * License: MIT
 * Copyright: NetYou
*/

add_action('get_header', 'my_filter_head');
function my_filter_head() { remove_action('wp_head', '_admin_bar_bump_cb'); }

function my_admin_css() {
        if ( is_user_logged_in() ) {
        ?>
        <style type="text/css">
            #wpadminbar {
                width: 47px;
                min-width: 47px;
                overflow: hidden;
                -webkit-transition: .4s width;
                -webkit-transition-delay: 1s;
                -moz-transition: .4s width;
                -moz-transition-delay: 1s;
                -o-transition: .4s width;
                -o-transition-delay: 1s;
                -ms-transition: .4s width;
                -ms-transition-delay: 1s;
                transition: .4s width;
                transition-delay: 1s;
            }
            
            #wpadminbar:hover {
                width: 100%;
                overflow: visible;
                -webkit-transition-delay: 0;
                -moz-transition-delay: 0;
                -o-transition-delay: 0;
                -ms-transition-delay: 0;
                transition-delay: 0;
            }
        </style>
        <?php }
}
add_action('wp_head', 'my_admin_css');

This is a great idea. But the way it animates in (at least in chrome) could be better.

Mmmm... on FireFox no Mac does't work very well... however good idea

Yes, it's more of a proof of concept. This could be much better with some simple javascript.
I might add some just to make it work properly cross browser.