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

The Calais Archive Tagger plugin automatically goes through your archives and tags every post you’ve written. The plugin uses the Open Calais API to perform semantic analysis of your post text and suggest tags. If a post already contains a suggested tag, that tag isn’t added, but other new tags found are. It takes about 5 minutes to tag 200 posts.
This plugin requires PHP 5 and the cURL library (both of which are available on most web hosts).
Also see <a href="https://wordpress.org/extend/plugins/calais-auto-tagger/" rel="ugc">WP Calais Auto Tagger</a> for suggesting tags as you write new posts.

WP Calais Archive Tagger

Goes through your archives and adds tags to your posts based on semantic analysis.

Auto-tags vs WP Calais Archive Tagger