Wordpress similar posts without plugin

gistfile1.php

<?php 
//=========================================================== SIDEBAR == Similar Posts
function similar_posts() { 
	echo "<li class=\"widget similar_posts\">";
	echo "<h3>Similar Posts</h3>";
	echo "<ul>";
			global $post;
			//stores the original post data
			$original_post = $post;

			//get the post category
			$category = get_the_category();
			$cat = $category[0]->cat_ID;

			//start loop and query stuff
			$args = array(
				'orderby' => 'rand',
				'cat' => $cat,
				'post__not_in' => array($post->ID),
				'showposts'=> 8,
				'caller_get_posts'=> 1
			);
			$related_posts_sidebar = new WP_Query($args);
			if ( $related_posts_sidebar->have_posts() ) :
				while ( $related_posts_sidebar->have_posts() ) : $related_posts_sidebar->the_post();
				?>
					<li><a title="<?php the_title(); ?>" href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
				<?php
				endwhile;
				wp_reset_query();
			endif;
	echo "</li>";
	echo "</ul>";
}