The AVH Extended Categories Widgets gives you three widgets for displaying categories.
Replacement of the default category widget to allow for greater customization.
A top categories widget. Shows the top X categories. This requires WordPress 2.8 or higher.
A Category Group widget. Shows categories from a group which is associated with the post/page that the visitor is looking at. Categories can be added to a group. Multiple groups can be made and every post/page can be associated with a different group. This requires WordPress 2.8 or higher.
More Info and Download:
http://blog.avirtualhome.com/wordpress-plugins/avh-extended-categories/
