Konstantin Sokolov

Performs automatic keyword extraction. Adds a keyword taxonomy for browsing and a keyword extraction metabox in the post editing page. 
Available in English and Mandarin.

Eventflow Keyword Extraction

Automatic keyword extraction for SEO enhancement and improved user experience.

GB071

ecreateinfotech

This plugin automatically converts keywords in a post/product title and category to tags upon saving. It includes a user-editable list of words you want the plugin to ignore
<ul>
<li>Converts keywords in post/product titles to tags</li>
<li>Converts keywords in post/product category to tags</li>
<li>Includes user-editable list of words to be ignored</li>
<li>Setting to enable for title and category separately</li>
<li>Setting to overwrite previous tag and consider only title and category while generating new one</li>
<li>Converts on publish and update.</li>
</ul>
Multi-blog adminstrators take note: this plugin is especially helpful if you’re building a community-based site where tagging is important and your bloggers are not always diligent about tagging.

Auto Tag Creator

Automatically creates tags from post/product title and category, on update or publish.

Eventflow Keyword Extraction vs Auto Tag Creator