P. Chandra

Automatically adds a META keywords tags with keywords within the html HEAD tags. The categories are used as keywords. 
Note: Works on single posts only as they are assigned categories.

Quick META Keywords

Automatically adds a META keywords tags with keywords within the html HEAD tags. The categories are used as keywords.

Devtard

This plugin automatically searches posts when they are published/imported/saved and adds new taxonomy terms (tags by default) if term names or related keywords from a user-created keyword list are found.
<h4>Features</h4>
<ul>
<li>Compatible with several post import plugins (<a href="https://wordpress.org/plugins/automatic-post-tagger/faq/" rel="ugc">FAQ #4</a>)</li>
<li>Supports custom taxonomies and post types; for example, APT is able to categorize posts (<a href="https://wordpress.org/plugins/automatic-post-tagger/faq/" rel="ugc">FAQ #6</a>), add tags/categories to WooCommerce products (<a href="https://wordpress.org/plugins/automatic-post-tagger/faq/" rel="ugc">FAQ #7</a>)</li>
<li>Supports UTF-8 characters, non-Latin and logographic alphabets (<a href="https://wordpress.org/plugins/automatic-post-tagger/faq/" rel="ugc">FAQ #5</a>), including Arabic, Chinese, Cyrillic etc.</li>
<li>Bulk tagging tool (with a scheduler) for processing multiple posts</li>
<li>Import/Export tools (CSV & JSON format support)</li>
<li>Configuration groups with custom rules for selected keyword sets; wildcard (regex) support for related keywords</li>
</ul>
See <a href="https://wordpress.org/plugins/automatic-post-tagger/screenshots/" rel="ugc">Screenshots</a> and <a href="https://wordpress.org/plugins/automatic-post-tagger/faq/" rel="ugc">FAQ</a> for more information.

Automatic Post Tagger

Adds relevant taxonomy terms to posts using a keyword list provided by the user.

Quick META Keywords vs Automatic Post Tagger