Das muss in die single.php: 
<strong>Ähnliche Beiträge:</strong>
<?php if (function_exists('fbbl_related_posts')) : ?>
<ul><?php fbbl_related_posts(); ?></ul>
<?php endif; ?>

Das muss in die functions.php:
// related post with wordpress-tags
// wordpress > 2.3
function fbbl_get_related_posts() {
	global $wpdb, $post;
	
	$now      = current_time('mysql', 1);
	$tags     = wp_get_post_tags($post->ID);
	$taglist  = "'" . str_replace("'", '', str_replace('"', '', urldecode($tags[0]->term_id))). "'";
	$tagcount = count($tags);
	
	if ( $tagcount > 1 ) {
		for ($i = 1; $i <= $tagcount; $i++) {
			$taglist = $taglist . ", '" . str_replace("'", '', str_replace('"', '', urldecode($tags[$i]->term_id))) . "'";
		}
	}
	$q = "SELECT DISTINCT p.*, count(t_r.object_id) as cnt FROM $wpdb->term_taxonomy t_t, $wpdb->term_relationships t_r, $wpdb->posts p WHERE t_t.taxonomy ='post_tag' AND t_t.term_taxonomy_id = t_r.term_taxonomy_id AND t_r.object_id  = p.ID AND (t_t.term_id IN ($taglist)) AND p.ID != $post->ID AND p.post_status = 'publish' AND p.post_date_gmt < '$now' GROUP BY t_r.object_id ORDER BY cnt DESC, p.post_date_gmt DESC LIMIT 5;";

	return $wpdb->get_results($q);
}
function fbbl_related_posts(){

	if ( function_exists('get_the_tags') && (get_the_tags()) ) {
		$related_posts = fbbl_get_related_posts() ;
	}
	
	if ( $related_posts ) {
		foreach ($related_posts as $related_post ) {
			$related_post_output = '<li>';
			$related_post_output = $related_post_output . '<a href="' . get_permalink($related_post->ID) . '" title="' . wptexturize($related_post->post_title) . '">' . wptexturize($related_post->post_title) . '';
			$related_post_output = $related_post_output . '</a></li>';
			echo $related_post_output;
		}
	} else {
		echo '<li>' . __('Keine &auml;hnlichen Beitr&auml;ge') . '</li>';
	}	
}
?>