by Fabio M. Giacomini on WordPress.org
The easiest way to add in every post page a different meta description tag, located in html head.

The admin option page
This plugin adds a new field in every post, where you can write a text that will be inserted in the related meta tag, located in the head section.
The main options page of the plugin, provides you with a default text area, where you can insert a default description that will appear in case the field in post page is empty.