Horizontal Main Categories, with Sub Categories displayed in the Vertical Menu.
Lite, CSS based.
Click Here for Online Demo in v4.2.2
Click Here for Online Demo in v4.4.2
The demo shows the technical workings of the nav - it isn't customised to give the full effect, but with customisation, can give an effect similar to:
The 4.4 demo opens up in a sub category and demonstrates the highlighting of the current category and also it working with sub, sub categories in X-Cart.
For X-Cart versions 4.1.x, 4.2.x, 4.3.x, 4.4.x and 4.5.x
(note functionality may vary slightly for each version)