Forum OpenACS Q&A: automatic text catgorization
Most of those seem to have been used so far primarily or only as spam filters, but there have definitely been other applications (Extracting the interesting posts from Usenet, for example.)
Someone or other here also wrote a college thesis doing automatic classification of text in an OpenACS system, but I never read it and now I don't remember who that was or where it is.
There was also some old discussion of OpenCyc, which is of course quite different.
"Williams K., R. A. Calvo and D. Bell. Automatic Categorization of Questions for a Mathematics Education Service. Artificial Intelligence in Education Conference. Sydney, Australia. July 2003"It can be found here: http://www.weg.ee.usyd.edu.au/people/rafa/papers/