function related_posts_shortcode( $atts ) {
    extract(shortcode_atts(array(
        'limit' => '5',
    ), $atts));

    global $wpdb, $post, $table_prefix;

    if ($post->ID) {
        $retval = '<ul>';
        // Get tags
        $tags = wp_get_post_tags($post->ID);
        $tagsarray = array();
        foreach ($tags as $tag) {
            $tagsarray[] = $tag->term_id;
        }
        $tagslist = implode(',', $tagsarray);

        // Do the query
        $q = "SELECT p.*, count(tr.object_id) as count
            FROM $wpdb->term_taxonomy AS tt, $wpdb->term_relationships AS tr, $wpdb->posts AS p WHERE tt.taxonomy ='post_tag' AND tt.term_taxonomy_id = tr.term_taxonomy_id AND tr.object_id  = p.ID AND tt.term_id IN ($tagslist) AND p.ID != $post->ID
                AND p.post_status = 'publish'
                AND p.post_date_gmt < NOW()
            GROUP BY tr.object_id
            ORDER BY count DESC, p.post_date_gmt DESC
            LIMIT $limit;";

        $related = $wpdb->get_results($q);
        if ( $related ) {
            foreach($related as $r) {
                $retval .= '
    <li><a title="'.wptexturize($r->post_title).'" href="'.get_permalink($r->ID).'">'.wptexturize($r->post_title).'</a></li>
';
            }
        } else {
            $retval .= '
    <li>No related posts found</li>
';
        }
        $retval .= '</ul>
';
        return $retval;
    }
    return;
}
add_shortcode('related_posts', 'related_posts_shortcode');

[related_posts]