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.

dangrossman

With the Calais Auto Tagger plugin, you’ll never have to think of tags for your posts again. The plugin uses the Open Calais API to perform semantic analysis of your post text and suggest tags for you. Add them to your post with just a click.

WP Calais Auto Tagger

The plugin performs semantic analysis of your posts to suggest tags using Open Calais.

Auto-tags vs WP Calais Auto Tagger