Sort posts by popularity/page views

The WP Postviews plugin is one of the most used to record post views.

Then you can sort by amount by;

<?php if (function_exists('get_most_viewed')): ?>
<ul>
    <?php get_most_viewed(); ?>
</ul>
<?php endif; ?>

Or pass in the variables to the URL:

http://example.com/?v_sortby=views&v_orderby=desc

Or even run a custom query that sorts by the post_meta value.