Similar Posts – NO plugin

Put this in your single.php:

$tags = wp_get_post_tags($post->ID);
if ($tags) {
$first_tag = $tags[0]->term_id;
$args=array(
'tag__in' => array($first_tag),
'post__not_in' => array($post->ID),
'showposts'=>4,
'caller_get_posts'=>1
);
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) {
while ($my_query->have_posts()) : $my_query->the_post(); 
 // post content stuff here
endwhile;
wp_reset_query();
}
}