my-sites-search.php

<?php
/*
 * Plugin Name: My Sites Search
 * Plugin URI: trepmal.com
 * Description:
 * Version:
 * Author: Kailey Lampert
 * Author URI: kaileylampert.com
 * License: GPLv2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * TextDomain: mss
 * DomainPath:
 * Network:
 */


add_action( 'admin_bar_menu', 'mss_admin_bar_menu' );
function mss_admin_bar_menu( $wp_admin_bar ) {
	$wp_admin_bar->add_menu( array(
		'parent' => 'my-sites-list',
		'id'     => 'my-sites-search',
		'title'  => '<input type="text" placeholder="Search sites" />',
	) );
}

add_action( 'admin_footer', 'mss_admin_footer' );
function mss_admin_footer() {
	// This should suck less, but alas...
	?>
	<style>
	#wp-admin-bar-my-sites-search {
		height: 34px;
	}
	</style>
	<script>
	jQuery(document).ready( function($) {
		$('#wp-admin-bar-my-sites-search input').keyup( function( ) {

			var searchValRegex = new RegExp( $(this).val(), 'i');

			$('#wp-admin-bar-my-sites-list > li.menupop').hide().filter(function() {

				return searchValRegex.test( $(this).find('> a').text() );

			}).show();

		});
	});
	</script>
	<?php
}