The "show/hide" categories menu:
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=872
is particularly good for SEO, as it does not clutter the VISUAL PAGE, but still populates ALL HTML PAGES with the full category tree.
For the human visitor, the sub-cat content remains "hidden" unless selected to view - resulting in an uncluttered menu sidebar.
For the search engines however, EVERY internal category link (in its established hierarchy) is there for them to navigate and spider, on EVERY generated page... Good SEO fodder!
ONE WARNING... Don't use this if you have a category depth exceeding 150 or so categories and sub categories. TOO MANY internal links can be interpreted as spammming and stuffing...