A sidebar widget that displays all tags on your site, formatted so that the most commonly used tags are the most visually prominent.
Add a description/blurb before a category listing, or make a post in a category always appear first.
An overview of tags, and how to add/remove them from posts.
An overview of tags & categories, and suggestions on when to use which taxonomy.
How to add a new post category while editing a post, or from the Posts -> Categories tool in the dashboard.