Epigrade

Add page title, meta description, keywords and robots to all post types and taxonomies. 
Plugin will automatically change page titles for posts, pages, custom posts, categories, tags and custom taxonomies to 
whatever you have entered. Also will inject meta fields in template head section. If however you leave fields empty 
default WordPress or theme behaviour will occur. 
This plugin also removes taxonomy description column from listing table.
You can use it in your theme or other plugin. It creates the following taxonomy and post meta fields: 
 _page_title, <code>_meta_description</code>, <code>_meta_keywords</code>, <code>_meta_robots</code>.
Example usage:
<pre><code>// Get page title value for taxonomy with ID $term_id:
$taxonomy_title = get_term_meta( $term_id, '_page_title', true );

// Get page title value for post with ID $post->ID:
$post_title = get_post_meta( $post->ID, '_page_title', true );
</code></pre>

WP Simple SEO Meta

Add page title, meta description, keywords and robots to all post types and taxonomies.

Erunafailaro

The plugin adds a meta-tag to the html-header-part of your blog. This meta-tag contains keywords that describe the contents of the html-page. Adding this kind of meta-tag is one part of the job of optimizing your blog for search engines. To learn more about search engine optimization, I recommend reading and understanding these articles:
<ul>
<li><a href="http://www.geek-notes.com/wordpress/25/optimize-wordpress-for-search-engines" title="About SEO at geek-notes.com" rel="nofollow ugc">about SEO at geek-notes</a></li>
<li><a href="http://www.devlounge.net/articles/plugins-for-a-search-engine-friendly-wordpress-blog" title="About SEO at devlounge.net" rel="nofollow ugc">about SEO at devlounge</a></li>
</ul>
<h3>Plugin Homepage</h3>
The complete documentation is located at my <a href="http://www.weinschenker.name/wptags-4-metakeywords/" title="Plugin-homepage" rel="nofollow ugc">website</a>

WPtags 4 MetaKeywords

This is a tiny, lightweight plugin I wrote to use the new native tagging from WordPress 2.3+ to automatically create a meta-keyword-tag for my blog.

WP Simple SEO Meta vs WPtags 4 MetaKeywords