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.

digitalemphasis

This plugin detects your ‘already existing tags’ into your post each time you create or edit/save one. The found tags will be automatically assigned.
<h4>Features</h4>
<ul>
<li>Easy configuration.</li>
<li>Allow or block manually added tags; the choice is yours.</li>
<li>You can choose if the plugin examines the title, the content or both.</li>
<li>You can activate a filter and select which categories will be affected and which ones will be ignored by the plugin.</li>
<li>Clean uninstall option: If this option is enabled, the plugin will leave absolutely no traces when uninstalling.</li>
<li>Visit <a href="https://digitalemphasis.com" rel="nofollow ugc">digitalemphasis.com</a> for more info.</li>
</ul>

Already Existing Tags

Looks for already existing tags within your posts.

Auto-tags vs Already Existing Tags