Kumbh Design Inc.

You can set meta tags like keyword, description. You can customize on each individual page/post as well.
<h3>Features</h3>
<ul>
<li>FREE Plugin.</li>
<li>One click to Install.</li>
<li>Simple and Easy to Use</li>
<li>Do not need to edit your theme’s core files.</li>
<li>Quick and supportive.</li>
</ul>

WP Simple Meta Tags

Allows you to set meta tags like keyword, description and you can customize on each individual page/post.

Philipp Speck

This plugin will make it possible to use tags as <code>meta keywords</code>. Under <code>Pages</code> you will find a new meta box called <code>Keywords</code>. 
By activation the plugin registrates a separate taxonomy which is independent from post tags. 
Today’s search engines don’t use <code>meta keywords</code> as a criteria at all, but who knows for what they will be good in future 😉 
For me tagging is necessary to bring pages in relation and gives me the possibility to save my tags redundant in the database.
Features
<ul>
<li>Use post tags as <code>meta keywords</code></li>
<li>Registrates own <code>taxnomy</code> for pages</li>
<li>No coding needed</li>
<li>Language support</li>
</ul>
Related Links
<ul>
<li><a href="http://www.typomedia.org/wordpress/plugins/wordpress-meta-keywords/" title="Homepage of WordPress Meta Keywords" rel="nofollow ugc">Homepage</a></li>
<li><a href="https://wordpress.org/extend/plugins/wordpress-meta-keywords/changelog/" title="Changelog for WordPress Meta Keywords" rel="ugc">Changelog</a></li>
<li><a href="https://wordpress.org/extend/plugins/wordpress-meta-keywords/faq/" title="FAQ for WordPress Meta Keywords" rel="ugc">Questions</a></li>
</ul>
Localization
<ul>
<li>German translation by <a href="http://www.destio.de/" title="Designstudio, Philipp Speck" rel="nofollow ugc">Designstudio, Philipp Speck</a></li>
<li>Slovak translation by Branco Radenovich</li>
</ul>
Note: 
Before installing or upgrading new plugins, please backup your database first!

WordPress Meta Keywords

This plugin gives you the ability to set Meta Keywords in WordPress. Under pages it will registrate a separate taxonomy for keywords.

WP Simple Meta Tags vs WordPress Meta Keywords