A small modification to this nice and neat contribution - I wanted to have a sum of all the products in the top category and its subcategories displayed after the top category name.

With the following corrections:
-correct count even if product is in multiple categories
-no longer counts disabled products

So if you don't want a product count behind the category name then use Christian's contribution below.

Installation:
1) Unzip in catalog/includes/boxes.
2) Edit catalog/includes/column_left.php and change 'categories.php' to 'top_categories.php'

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