This is just a little snippet of code I wrote to count the number of hits on a specific page. I only wanted it to work on single post pages so I included the is_single() conditional tag.

function count_page_hits() {
   if(is_single()) {
      global $post;
      $count = get_post_meta($post->ID, 'count_page_hits', true);
      $newcount = $count + 1;

      update_post_meta($post->ID, 'count_page_hits', $newcount);
   }
}
add_action('wp_head', 'count_page_hits');

All the code does it increase the count whenever the single post page is accessed. Now you will have a custom field with the number of times the page is hit. With this data, you can start to create a list of popular posts or even a graph to show you in your dashboard which posts have received the most views on your site.

Tutorials on how to do those things will come out shortly.