PSMAN

TagGator is an auto tagging plugin, Just write enter a few keywords representing your contents, TagGator will automatically scan your posts for these keywords, and tag them according to the results.
A Pro Version of TagGator is now available with the following features:
<ul>
<li>Auto Tagging: Your posts are tagged automatically when you create posts.</li>
<li>Autoblog friendly:Taggator works with most autoblogging solutions, tags are created automatically when the posts are generated.</li>
<li>Keywords To tags: Just write different keywords that represent you tags, Taggator will create the tags, and assign them.</li>
<li>Set & Forget: You only need to configure TagGator once. once done, you won’t have to worry about tags anymore</li>
</ul>
<a href="http://codecanyon.net/item/taggator-pro-wordpress-auto-tagging-plugin/1725033?ref=AngryByte" rel="nofollow ugc">Click here to learn more & get TagGator Pro</a>

TagGator

TagGator Automatically tag to your posts so you dont have to! Tag your posts based on their contents automatically.

jfoucher

This plugin uses the yahoo.com API to find the most relevant keywords from the content of your post, and then adds them as tags.
The tagging is fully automatic, so if you’re using a plugin like feedwordpress to display RSS feeds on your blog as posts, everything will get done as the feed posts are published. No user intervention necessary!

Auto-tags

Automatically add relevant tags to new posts.

TagGator vs Auto-tags