Fabio M. Giacomini

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.

Single Post Meta Description

The easiest way to add in every post page a different meta description tag, located in html head.

Philipp Speck

This plugin will give you the ability to use the <code>excerpt</code> as <code>meta description</code> for any post or page. From now you are able to control the description of your site which will be displayed on the search engine results pages. 
By activation the plugin enables the excerpt for static edit pages in admin area.
Related Links
<ul>
<li><a href="http://www.typomedia.org/wordpress/plugins/wordpress-meta-description/" title="Homepage of WordPress Meta Description" rel="nofollow ugc">Homepage</a></li>
<li><a href="https://wordpress.org/extend/plugins/wordpress-meta-description/changelog/" title="Changelog for WordPress Meta Description" rel="ugc">Changelog</a></li>
<li><a href="https://wordpress.org/extend/plugins/wordpress-meta-description/faq/" title="FAQ for WordPress Meta Description" rel="ugc">Questions</a></li>
<li><a href="http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35264#1" title="Create good meta descriptions" rel="nofollow ugc">Information</a></li>
</ul>
Note: 
Before installing or upgrading new plugins, please backup your database first!

WordPress Meta Description

With this plugin you can control the text which will be shown as description on the search engine results pages.

Single Post Meta Description vs WordPress Meta Description