JFrankParnell

This plugin lists all your posts and displays checkboxes for all your currently registered categories. You can check or uncheck categories and then hit any one of the submit buttons and your posts will be put into the categories you’ve chosen
<ul>
<li>I’ve used this for 200 posts. I dont really know what will happen if you try to change 2000 posts at a time.</li>
<li>I am going to try to implement a paging feature so that huge post lists wont crash anything.</li>
<li>The compatability i’ve listed simply means it’s what i’ve used it on. This will probly work on any >3 wp version.</li>
<li>If it works for you, please click ‘works’ on the right over there. If it doesnt, start a thread on the support tab 🙂</li>
</ul>
<h3>Arbitrary section</h3>
Well, this is pretty abitrare.

Mass set Categories

Otto Kekäläinen

Viljami Kuosmanen

Onni Hakala

Currently WordPress doesn’t allow you to add custom bulk actions. See codex. Adding them is super easy with this plugin.
This is based on solution found here: http://www.skyverge.com/blog/add-custom-bulk-action/ 
This plugin just makes it more easier to use.
Plugin adds a class named Seravo_Custom_Bulk_Action which you can use to easily define custom bulk actions. 
See documentation and examples here: https://github.com/Seravo/wp-custom-bulk-actions/

Custom Bulk Actions

Currently Wordpress doesn't allow you to add custom bulk actions. Adding them is super easy with this plugin.

Mass set Categories vs Custom Bulk Actions