The Single Post Template plugin adds the ability for your theme to include “Post Templates” in much the same way you can add “Page Templates”, allowing you to choose (via a simple dropdown) which post template you want to use, on a per-post basis.
All you need to do, after installing and activating the plugin, is create one or more “Post Templates” in your theme’s folder, and when creating or editing a post, choose the post template that you would like to use. If no post template is selected, the default template will be used.