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.

dconsorte

This WordPress plugin is free, and makes tagging your posts a snap. It’s a HUGE TIME-SAVER.
After it’s installed, you’ll see a checkbox labeled “Autopopulate Tags” just above the Publish or Update button on the post editing page in your website admin. Simply check the box, then either click “Save Draft”, “Publish” or “Update”.
The plugin will scan the title and content of your blog post and compare it to your entire list of keyword tags. If they are present in your content, they’ll be added to the post automatically. You may then either delete irrelevant tags, or add new ones, which will be part of the next scan you run.

Autotag Posts by SHIFT1

A plugin to automatically add tags to a blog post by scanning the post content and adding tags for all included phrases.

Auto Tag Generator vs Autotag Posts by SHIFT1