The WPEssence Bulk Categories plugin allows for easily creating multiple categories at once, including slugs and category parents.
As of the 14th of march, 2013, this plugin is no longer supported as it has been superseded by the BulkPress plugin. The BulkPress plugin does everything the WPEssence Bulk Categories plugin does but adds enhanced functionality to adding terms and allows you to add other types of content, such as posts.
The WPEssence Bulk Categories plugin will no longer be supported, and you are strongly encouraged to deactivate it on your website and install the BulkPress plugin instead.
Update: As of version 1.1, WPEssence Bulk Categories also allows you to create tags, other custom taxonomies and even taxonomies registered by your plugin or theme!
Have you ever had to create a category for every single city in the Netherlands? I know I have. It can take quite some time to manually add tens or even hundreds of categories through the WordPress admin panel, and that's where this plugin comes in.
The WPEssence Bulk Categories plugin allows you to easily add multiple categories to your WordPress website, without manually adding them one by one. The plugin features full category paths, meaning you can add top categories as well as category children. WPEssence Bulk Categories will not do anything to the front-end of your website, but just add a menu item under "Posts" in the sidebar of the admin panel, where you can add as many categories as you want in one go.
WPessence Bulk Categories is not limited to standard categories, it features other taxonomies such as tags, but also taxonomies registered by your plugin or theme. Just select the taxonomy you wish to create at the top of the Bulk Categories admin panel page!
Custom slugs are also possible for every created category, as well as the option to add all categories you want to create under the same parent category, so you won't have to type out that category name in the category path every time. You can also choose not to create parent categories which do not already exist, to make sure there won't be a new branch of categories if you accidentally typed the wrong category name in one of the paths.