scribu

This plugin is outdated. Use <a href="https://wordpress.org/extend/plugins/term-management-tools/" rel="ugc">Term Management Tools</a> instead.

Merge Tags

Adds a new form on the tag management screen that lets you merge and replace tags and categories.

Evan Herman

Tracy Levesque

yikesitskevin

karmadharma

Order all of the taxonomy terms on your site with a simple to use, intuitive drag and drop interface. The plugin works for WordPress core taxonomies — Categories and Tags — and any custom taxonomies you have created.
Activate the plugin, enable your taxonomy on the settings page, and drag and drop the taxonomies into the desired position. It couldn’t be easier.
On the front end of the site your taxonomy terms will display in the order set in the dashboard.
Requires WordPress 4.4 or later due to the use of the term meta. 
Query Usage
<ul>
<li>If you’re trying to query for taxonomy terms (e.g. using <code>WP_Query</code> or functions like <code>get_terms()</code>), and you’d like them to be returned in the order specified by the plugin, you need to add the tax_position parameter in your call. For example: <code>'meta_key' => 'tax_position'</code> and <code>'orderby' => 'tax_position'</code>. Thanks to @certainlyakey on GitHub for pointing this out.</li>
</ul>

Simple Taxonomy Ordering

I think this is the easiest/best custom Taxonomy order plugin, as it keeps it within the same WordPress admin screen, does not create its own page. 

Simple

Works like a charm - use it

The best drag&drop ordering of terms so far.

Thanks so much!!

Quickly and easily reorder taxonomy terms with an easy to use and intuitive drag and drop interface.

Merge Tags vs Simple Taxonomy Ordering