antonioandra.de

Enables your blog readers to suggest new post tags or upvote/downvote existing ones. 
Downvoted tags are eventually removed. 
A few more features are on the works as is the expansion of this readme document.
<h3>RoadMap</h3>
Add some frontend feedback; 
Add support for custom taxonomy tagging; 
Synonym dictionary; 
Better documentation, ui and automatic integration.

Author: António Andrade

Enables your blog readers to suggest new post tags or upvote/downvote existing ones.

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.

Author: António Andrade vs Auto-tags