“This [wordpress] plugin is simply intended as an example of how to implement tag and category intersections.”
#categories
TDO Tag Fixes
Plugin that allows you (amongst other things) to get a feed/page in WordPress that shows you all posts tagged with a specific tag (or set of tags) *within a certain category*.
Bulk Category Manager for WP
Apparently allows you to modify the category assignments for lots of posts at once in WP.