Folding Category Plugin (FoCaL)
As your blog gets more and more posts, inevitably you will create more categories to cope. Eventually your categories will take up more room on your screen, and scroll across many pages. This plug-in provides a Folding Category List (FoCaL) for WordPress version 2.3 and above which allows categories to be expanded and collapsed to save room on your sidebar.
On the plugin homepage you can also find full documentation as well as a list of more demo sites.
One of the main features of this WordPress plugin is that a prioritised list dictates which category is expanded. When viewing single posts which are assigned to multiple categories the default behaviour of WordPress is to select the category with the lowest numerical id. This plugin allows a priority list in which you specify the order that categories are expanded.
This plugin also allows expansion and highlighting of the list when viewing a page. Simply create a custom field for the page called “category” and give it a value equal to the numerical ID of the category to expand.