Please download the plugin from here, hosted in the official Zen-Cart site.
This Zen-Cart plugin creates a new side-box which shows all the active categories as well as the sub categories. At any time all the active sub categories are also loaded recursively and not only when a parent category is visited as in the regular category side box. The sub categories are shown as slide out sections on rolling over their respective parent sections.
Installation and Modification
All the files used are new files and hence no over writing of core Zen Cart files are needed. After placing all the files maintaining the folder structure, the side box needs to be activated from the admin back end. Of course the original category side box needs to be deactivated also.
For modification of design please play with the provided CSS.
Added an updated version which supports Zen Cart 1.5.0 and is PHP 5.3 compatible. Also fixes the blank sidebox bug some users are getting.