slaptijack

Suggest Tags creates a list of tag suggestions based on the content of the post you are editing. Since the Yahoo! Term Extraction API is going to be shutdown at the end of August 2009, this plugin uses the term extraction API from tagthe.net.

Suggest Tags

Suggest Tags uses the tagthe.net tagging web service to generate a list of tag suggestions.

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.

Suggest Tags vs Auto-tags