fmzac

This plugin automatically converts keywords in a post title to tags upon saving. It includes a user-editable list of words you want the plugin to ignore, which by default includes the more obviously-useless words like “I” or “wasn’t.” You can also reset the list back to defaults by deleting/empty the current list.
<ul>
<li>Converts keywords in post titles to tags</li>
<li>Includes user-editable list of words to be ignored</li>
<li>Ignore list can be reset to default at any time</li>
<li>Converts on save, not publish.</li>
<li>Does not convert if there are already tags assigned.</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 Generator

Automatically creates tags from post title, on update or publish.

Thamizhchelvan

You can use this plugin to generate keywords or tags based on a given keyword.

WP Tag Generator

Will generate keywords based on the input, you can specify the form field to use the fetched keyword.

Auto Tag Generator vs WP Tag Generator