viterbiSearcher is right in stating that one of the commonplace uses of ontology is classification systems. What is missing is the fact that the internet has made this a very active field.
Most search engine companies employ ontologists to dream up their classification systems. The Open Directory Project is an attempt to publish a directory of the web (with categories, like Yahoo) by volunteers. Go there and put "ontology" into their search engine and you'll find that ontology is indeed a hot topic these days.