Just like pages and posts have custom fields, so should categories and tags (terms)
Here are some examples for when this would be useful:
*Displaying category images or tags, i.e. an image that is displayed whenever a category or tag archive is viewed. The path to the image file could be stored in the category or tag custom field, which would be retrieved by a template tag in the theme.
*Associating banner ads with category archive or tag pages. The banner ad code or configuration parameters could be stored in a custom field. This would also be retrieved and displayed by code within the theme.
*Tracking the number of times that a category or tag archive page is viewed, much in the similar way that this is done by other post plugins like WP-PostViews.
At the moment, there is a plugin "WP Category Meta" that implements custom fields for terms, but it has not been updated to work with WordPress 3.2
I think the functionality of this plugin should be formally implemented in the WordPress code base.
This plugin creates an additional table for terms meta, plus some PHP functions to update and access the terms meta. Their syntax is virtually identical to the ones used for post meta.
The plugin can be found here: