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.

Jackson Whelan

Taxonomy Templates allows user selection of a theme’s available taxonomy templates, much like how pages can select their template.
When additional templates are available, a drop down appears on the edit Category, Tag and any custom taxonomy pages.

Taxonomy Templates

Taxonomy Templates allows user selection of a theme's available taxonomy archive templates, much like how pages can select their template.

Custom-Single vs Taxonomy Templates