Aswin Anand

With this plugin, assigning multiple categories to one or more posts is a 
breeze. Pop the plugin’s PHP file to your wordpress plugin directory, 
activate it and click on “Assign Categories” under the Manage menu. 
The page will show the list of available categories, followed by the 
available blog posts. You can select the categories, select the required 
posts and then click on “Assign Categories” at the bottom of the page. 
Now, all your posts will be assigned the new categories.
Plugin Page: <a href="http://www.aswinanand.com/2008/10/bulk-assign-categories-to-multiple-posts/" rel="nofollow ugc">http://www.aswinanand.com/2008/10/bulk-assign-categories-to-multiple-posts/</a>

Assign Categories

Assign one or more categories to multiple posts in a single shot, with or without preserving existing categories.

Landon

After you’ve been using your website for a little while you probably have some categories, tags, or custom terms you once used, no longer use, but don’t want to wade through. WP Term Manager allows you to hide these terms.
The settings can be found under Tools > WP Term Manager. Select which terms you want hidden and click “Save Changes”. That’s it!

WP Term Manager

WP Term Manager helps users clean up or hide terms for easier administration.

Assign Categories vs WP Term Manager