Stemming is something that\'s needed in tagging systems. I use delicious, and I don\'t have time to manage and prune my tags. I\'m a bit more careful with my blog, but it
Lucene has a stemmer in, I believe (and IIRC it lets you use your own one if you want).
EDIT: Just checked, and Lucence refers to the Snowball site which is an open source stemming library as far as I can tell.