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.

Erick Hitter

Creates dropdown lists of non-hierarchical taxonomies (such as <code>post tags</code>) as an alternative to term (tag) clouds. Multiple widgets can be used, each with its own set of options.
Numerous formatting options are provided, including maximum numbers of terms, term order, truncating of term names, and more.
Using the <code>taxonomy_dropdown_widget()</code> function, users can generate dropdowns for use outside of the included widget.
Only use version 2.2 or higher with WordPress 4.2 and later releases. WordPress 4.2 changed how taxonomy information is stored in the database, which directly impacts this plugin’s include/exclude term functionality.
This plugin was formerly known as the <code>Tag Dropdown Widget</code>. It was completely rewritten for version 2.0.
Follow and contribute to development on GitHub at https://github.com/ethitter/Taxonomy-Dropdown-Widget.

Taxonomy Dropdown Widget

Creates a dropdown list of non-hierarchical taxonomies as an alternative to the term (tag) cloud. Formerly known as Tag Dropdown Widget.

Assign Categories vs Taxonomy Dropdown Widget