Wordpress Plugins
Term Management Tools for WordPress
Organizing your tags and categories when you are just starting is not hard. It gets a lot of work as your site grows though. Term Management Tools for WordPress is just the plugin for the job. It lets you combine multiple terms into one, convert terms and set parents for each term.
This plugin works with tags, categories, and custom taxonomies. It is available as a free download. You can find out more about it here.