How to display the top 5 popular links in the header?

Put this in your functions.php file

function k99_post_hits( $id, $action ) {
        $dl_HitMetaField    = '_dl_post_hits'; // hidden Custom field that stores the views
        $dl_PostHits    = get_post_meta($id, $dl_HitMetaField, true);
        switch ($action) {
        case 'count' :
        if ( $dl_PostHits =='' ) {
            $dl_PostHits = mt_rand(10,20);//just for debug-test - remove for real count
            delete_post_meta( $id, $dl_HitMetaField);
            add_post_meta( $id, $dl_HitMetaField, $dl_PostHits );
            }
            $dl_PostHits++;
            update_post_meta( $id, $dl_HitMetaField, $dl_PostHits );
            break;
        case 'display' :
        echo 'this page was viewed: ' , $dl_PostHits ;
} 
}

and this :

function k99_most_popular() {
global $wp_query, $post, $paged, $post_count;

    // YOUR QUERY
    $query_args = array (
        'meta_key' =>'_dl_post_hits', 
        'orderby' => 'meta_value',
        'order' => ASC ,
        'posts_per_page' => 5 // how many we want ?
    );    

    // SAVE CURRENT QUERY
    $temp = $wp_query;
    $wp_query= null;    

    // CREATE NEW QUERY
    $wp_query = new WP_Query();
    $wp_query->query($query_args);      
$output="<span>";
        // THE LOOP, DO WHAT YOU HAVE TO DO HERE
        while ($wp_query->have_posts()) : $wp_query->the_post(); {   
$output .= '<li><a href="' .get_permalink() .'" title="' . get_the_title(). '"/>'. get_the_title() .'</a></li>';
}
        endwhile;  
$output .= '</span>';       
echo $output;

        // SWAP BACK THE PREVIOUS QUERY
        $wp_query = null; 
        $wp_query = $temp;
        wp_reset_query();   
}

then , in the template file you want to track (typically post &/or single &/or page)

put this in side the loop (preferably as first function before any output):

<?php k99_post_hits( $post->ID, 'count' );  //adding post counter ?>

then – wherever you would like to DISPLAY the count, put this :

<?php k99_post_hits( $post->ID, 'display' );  //adding post counter ?>

to get most “viewed”

 <?php k99_most_popular(); //getting post list by views ?>