Tor N. Johnson

This plugin enables themes to specify a different single.php template based on post category. As posts can have multiple categories assigned, it uses the following to select which possible custom template to use:
1) Categories directly assigned 
 — Check for extant single-CATEGORYID.php files and prioritize based on category order 
2) Parents of categories 
 — Parents are checked based on category order of the directly assigned categories.
Note, the above assumes that there’s a category order plugin installed. If the categories are NOT in a specific order AND there are multiple categories assigned per post then the template selection will be left up to the server. Naturally, if there is only category assigned per post (or only one possible template per post), then there will be no issue.
<h3>Requirements</h3>

Custom-Single

Enables themes to support custom single.php templates per category in the form single-CATEGORYID.php.

Stephen AfamO

This plugin allows you to use any HTML or PHP file as the template for any page or post.
Simply upload the file and select it. 
You can upload custom js and css files into the media library and link to them from the HTML file.
Options:
<ul>
<li>Overwrite All: You overwrite the entire theme and use your custom file</li>
<li>Overwrite Content: Keeps the header, footer, sidebar, e.t.c. Simply overwrites the body of the page or post</li>
<li>Above Content: Your custom content is simply added to the top of the page content</li>
<li>Below Content: You custom content is placed just beneath the page content.</li>
</ul>
<h4>Adding support for custom post types</h4>
By default the pulugin works with pages and posts, however, go to the settings to enable it on any other registered post type.
use the <code>hppp_post_types</code> filter to add more post types.
Like this:
<pre><code> public function post_type_modify ($post_types) {
 $post_types[] = 'custom_post_type';
 return $post_types;
 }

 add_filter( 'hppp_post_types', 'post_type_modify' );
</code></pre>

Custom HTML/PHP Post Templates

I use this on a couple sites I own and it works wonderfully.
I hope you can find it useful. 

Works like a charm

Use your HTML or PHP files for any page or post.

Custom-Single vs Custom HTML/PHP Post Templates