$tags = wp_get_post_tags($post->ID);
if ($tags) {
  $first_tag = $tags[0]->term_id;
  $args=array(
    'tag__in' => array($first_tag),
    'post__not_in' => array($post->ID),
    'showposts'=>5,
    'caller_get_posts'=>1
   );
  $my_query = new WP_Query($args);
  if( $my_query->have_posts() ) {
	echo '<ul>';
    while ($my_query->have_posts()) : $my_query->the_post();
      echo '<li><a href="'.the_permalink().'" rel="bookmark">'.the_title().'</a></li>';
    endwhile;
	echo '</ul>';
  }
}