Some of the directories in my collection of community web sites (like shopping) just weren't fitting well in a hierarchical tree; So I am trying a new format for these directories. Right now my idea is to use a format with subdirectories listed in a navigation block at the top of the page for directories with a small sub tree or that are cleanly delineated (like arts). I would then use a side bar navigation for directories with complex subdirectories.
I am not sure if mixing navigation structures that will work very well. But the sites are largely about experimentation. The side bar navigation is a lot easier to write in pure CSS format. A two column navigation bar is easiest to implement with a two column table.