By default, the categories widget in WP E-commerce is fully expanded. If you have a large number of categories and subcategories this can be a bit too much. The WP E-commerce Expanding Categories plugin wraps the code that is discussed at http://ken.ph/wp-e-commerce-expandablecollapsible-menu/ in a WordPress plugin so that no editing of the theme is necessary, and turns the categories widget into an expandable/collapsible menu.