With the YQL Auto Tagger plugin, you’ll never have to think of tags for your posts again. The plugin uses the Yahoo Query Language to perform semantic analysis of your post text and suggest tags for you. Add them to your post with just a click.
This plugin requires PHP 5 and the cURL library (both of which are available on most web hosts).
Based on the Open Calais Auto Tagger by Dan Grossman – http://www.dangrossman.info