Following code will display a list of 5 posts selected randomly by using the MySQL RAND() function for the orderby parameter value:

<ul><li><h2>A random selection of our Posts</h2>
<ul>
<?php
$rand_posts = get_posts('numberposts=5&orderby=rand');
foreach( $rand_posts as $post ) :
?>
<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endforeach; ?>
</ul>
</li></ul>