Looking for an ultra-smart tag or category builder for phrases

I’ve seen these on sites before, tag and/or category builders that can read and parse posts, find the most relevant phrases, and turn those phrases into tags and categories.

I am looking for such a beast, either free or commercial. I have tens of thousands of posts I need to tag and/or categorize and I’d like to have a good quality tag/category set (and I can’t see doing it all by hand — I did that once for another project and never, ever want to do it again).

Thankee all!