Add filter menu to admin list of posts (of custom type) to filter posts by custom field values

Simple to do, first create the dropdown with just the meta values you want and then catch the submit of the filter, just change POST_TYPE to the name of your post type and META_KEY to the name of your meta key:

<?php
/*
Plugin Name: Admin Filter BY Custom Fields
Plugin URI: http://en.bainternet.info
Description: answer to http://wordpress.stackexchange.com/q/45436/2487
Version: 1.0
Author: Bainternet
Author URI: http://en.bainternet.info
*/

add_action( 'restrict_manage_posts', 'wpse45436_admin_posts_filter_restrict_manage_posts' );
/**
 * First create the dropdown
 * make sure to change POST_TYPE to the name of your custom post type
 * 
 * @author Ohad Raz
 * 
 * @return void
 */
function wpse45436_admin_posts_filter_restrict_manage_posts(){
    $type="post";
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }

    //only add filter to post type you want
    if ('POST_TYPE' == $type){
        //change this to the list of values you want to show
        //in 'label' => 'value' format
        $values = array(
            'label' => 'value', 
            'label1' => 'value1',
            'label2' => 'value2',
        );
        ?>
        <select name="ADMIN_FILTER_FIELD_VALUE">
        <option value=""><?php _e('Filter By ', 'wose45436'); ?></option>
        <?php
            $current_v = isset($_GET['ADMIN_FILTER_FIELD_VALUE'])? $_GET['ADMIN_FILTER_FIELD_VALUE']:'';
            foreach ($values as $label => $value) {
                printf
                    (
                        '<option value="%s"%s>%s</option>',
                        $value,
                        $value == $current_v? ' selected="selected"':'',
                        $label
                    );
                }
        ?>
        </select>
        <?php
    }
}


add_filter( 'parse_query', 'wpse45436_posts_filter' );
/**
 * if submitted filter by post meta
 * 
 * make sure to change META_KEY to the actual meta key
 * and POST_TYPE to the name of your custom post type
 * @author Ohad Raz
 * @param  (wp_query object) $query
 * 
 * @return Void
 */
function wpse45436_posts_filter( $query ){
    global $pagenow;
    $type="post";
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }
    if ( 'POST_TYPE' == $type && is_admin() && $pagenow=='edit.php' && isset($_GET['ADMIN_FILTER_FIELD_VALUE']) && $_GET['ADMIN_FILTER_FIELD_VALUE'] != '') {
        $query->query_vars['meta_key'] = 'META_KEY';
        $query->query_vars['meta_value'] = $_GET['ADMIN_FILTER_FIELD_VALUE'];
    }
}