Simon Wheatley

Provides a drop-down to select different templates for posts from the post edit screen. The templates are defined similarly to page templates, and will replace single.php for the specified post. This plugin will NOT switch the templates for the different posts in a listing page, e.g. a date or category archive, it will only affect the template used for single posts (i.e. you can choose a template which is not single.php).
Post templates, as far as this plugin is concerned, are configured similarly to <a href="https://codex.wordpress.org/Pages#Creating_Your_Own_Page_Templates" rel="nofollow ugc">page templates</a> in that they have a particular style of PHP comment at the top of them. Each post template must contain the following, or similar, at the top:
<pre><code><?php
/*
Template Name Posts: Snarfer
*/
?>
</code></pre>
First note: Page templates use “Template Name:“, whereas post templates use “Template Name Posts:“.
Second note: You must have the custom post template files in your theme in the same directory/folder as your index.php template file, not in a sub-directory/sub-folder.
<h3>Developers</h3>
If you want to implement the custom post templates on a custom post type, you can use the <code>cpt_post_types</code> filter, here’s an example below of adding the custom post template selector and metabox to the “Movie” and “Actor” custom post types. This code can be added to a plugin or to the <code>functions.php</code> file in your theme.
<pre><code>/**
 * Hooks the WP cpt_post_types filter 
 *
 * @param array $post_types An array of post type names that the templates be used by
 * @return array The array of post type names that the templates be used by
 **/
function my_cpt_post_types( $post_types ) {
 $post_types[] = 'movie';
 $post_types[] = 'actor';
 return $post_types;
}
add_filter( 'cpt_post_types', 'my_cpt_post_types' );
</code></pre>

Custom Post Template

Provides a drop-down to select different templates for posts from the post edit screen. The templates replace single.php for the specified post.

Jamie O

This plugin makes category templates be selectable by drop-down list from the manage > categories screen of the admin console. It makes the selection / use of category and post templates as easy as page templates. It also includes configuration options that allow you to configure whether sub-categories inherit templates from their parent if none are specified.
Features:
<ul>
<li>
You can now use one category template for multiple categories without identical copies of the category-##.php
</li>
<li>
All category / post templates and settings are relative to the current theme applied to WordPress
</li>
<li>
If you select a different theme, the entries will remain in the database in case you want to revert back.
</li>
<li>
If you delete a category, all records (regardless of theme / version) will be deleted.
</li>
<li>
The Sub-Category Inheritance option applies for both category templates and post templates.
</li>
<li>
Now supports parent / child theme configurations.
</li>
</ul>

Idealien Category Enhancements

Modification to the category and post template logic to behave like pages (selectable through admin)

Custom Post Template vs Idealien Category Enhancements