Stephanie Leary

This plugin applies all content filters to posts and comments and saves them back to the database. This is useful if you have been using Textile or Markdown (for example) and you want to switch to plain HTML.
<h3>Notes</h3>
USE THIS PLUGIN WITH CAUTION. If the results are not what you want, the damage will be irreparable unless you take the following precautions.
You should back up your database first — and be sure that you know how to restore from the backup!
If you are have plugins that allow you to use short codes — e.g. [thing] — you should deactivate them before running this conversion. Otherwise, that markup too will be converted to its HTML equivalent. (The built-in WordPress [gallery] tag should be fine.)
Once your conversion is complete, you should deactivate this plugin and reactivate your short code plugins.

Mass Format Conversion

Applies all content filters to posts and comments and saves them back to the database.

Arno Esterhuizen

When you add text to the category description textarea and save the category, WordPress runs content filters that strips out all but the most basic formatting tags.
This plugin disables those filters for roles with the necessary permissions. Any html code you add to the category description will not be stripped out.
This plugin does not do anything other than disable the filters. It does not protect you from entering invalid HTML, nor does it help you create WYSIWYG HTML. You can use the post or page composing screen to help you create the text and formatting. Switch to the ‘code’ tab and copy the HTML code into the category description field.

Allow HTML in Category Descriptions

Great Plugin

This plugin allows you to use unfiltered HTML in your category descriptions by disabling selected WordPress filters.

Mass Format Conversion vs Allow HTML in Category Descriptions