Vito Tardia

With Simple SEO you can set your custom content for the “keywords” and “description” meta tags in each post or page using the standard WordPress metabox interface.
You can also set default values for each meta tag which will be displayed in all your pages, although this is not a SEO best practice.
<h4>Available languages</h4>
<ul>
<li>English</li>
<li>Italian</li>
<li>Romanian by Web Geek Science (<a href="http://webhostinggeeks.com/" rel="nofollow ugc">Web Hosting Geeks</a>)</li>
<li>Serbo-Croatian by Borisa Djuraskovic (<a href="http://www.webhostinghub.com/" rel="nofollow ugc">Webhostinghub</a>)</li>
<li>Ukrainian by Michael Yunat (<a href="//getvoip.com/blog" rel="nofollow ugc">http://getvoip.com</a>)</li>
<li>Indonesian by Jordan Silaen (<a href="//chameleonjohn.com" rel="nofollow ugc">http://chameleonjohn.com</a>)</li>
</ul>
<h3>Manual Database Update</h3>
In order to update your database you can run the following queries using your favorite MySQL admin tool, changing your table prefix if needed (a backup is strongly recomended).
<pre><code>UPDATE wp_postmeta SET meta_key = '_sseo_meta_keywords' WHERE meta_key = 'meta_keywords';

UPDATE wp_postmeta SET meta_key = '_sseo_meta_description' WHERE meta_key = 'meta_description';
</code></pre>

Simple SEO Pack

Simple SEO is a quick way to add HTML meta tags to your site and pages using WP integrated custom fields feature.

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.

Simple SEO Pack vs WordPress Meta Keywords