How to customize tag and category post listings to show introductory content?

You don’t need a plugin for that. You have at least two options:

1. Take advantage of the Category/Tag description field, to create content to output either on a category.php/tag.php template file, or else in archive.php or index.php inside an if ( is_category() )/if ( is_tag() ) conditional.
2. If you really need to customize your “introductory” content, create category-foo-bar.php template files, wherefoo-bar corresponds to the category slug (i.e. for category Foo Bar), and put in whatever content you need/want. Wash, rinse, and repeat for each category.

EDIT

One other solution: add a Dynamic Sidebar (i.e. Widget area) to the top of your Category archive index, and then use Text Widgets (with conditional logic, e.g. via the Widget Logic Plugin) to add whatever “introductory content” you want to add.