Easily apply templates to your category archive pages and posts with this plugin.
Valid templates are files placed your theme folder with the following code at the beginning of them:
For Post Templates: Template Name: Blog Template
For Archive Templates: Archive Template: Blog Template
Through this plugin’s options panel, you can choose which templates are applied to categories and posts. Even choose their priority over one another. You can even apply a template to an individual post. 
Please <a href="http://aarongloege.com/blog/web-development/wordpress/plugins/wordpress-plugin-category-templates/#respond" rel="nofollow ugc">report any bugs</a> you come across.
Enjoy!

Category Templates

Give your categories and posts their own individual templates.

Christopher Finke

Formategory allows you to customize posts based on their categories. Just define a template for a category, and all of the posts in that category will be displayed according to the template.
Use the buttons in the top-right meta box to insert placeholders for the post’s content; {{ the_content }} will be replaced by the post’s content, and {{ the_title }} will be replaced by the post’s title. Select some categories for the template, and after you save it, any posts in those categories will be instantly templated.
The “Category Templates” page in your dashboard allows you to manage your templates just like you manage your posts and pages.

Category Templates vs Formategory