Zen Cart automatically adds product numbers to each of your products. Here's some super easy ways to make those numbers more visible.

This is also a good tutorial introducing how one edits the product page

Add this code to includes/templates/YOUR_TEMPLATE/templates/tpl_product_info_display.php wherever you like (we chose right under the title, line 62)

  ID: <?php echo $_GET['products_id']; ?><br />

in includes/modules/meta_tags.php
Find line 275

define('META_TAG_TITLE', str_replace('"','',$review_on . $meta_products_name . $meta_products_price . PRIMARY_SECTION . TITLE . TAGLINE));

and replace it with

$meta_products_name .= " ID: " . $product_info_metatags->fields['products_id'];
define('META_TAG_TITLE', str_replace('"','',$review_on . $meta_products_name . $meta_products_price . PRIMARY_SECTION . TITLE . TAGLINE));