I've been using Spamassassin to check my email for spam for ages now. And it has worked pretty well on and off. Recently, people on one of the mailing lists I frequent have been discussing something called CRM114 (since the author of it is on the list). Last night I finally sat down and read up on it and tried installing it. After one false try and a little bit of fiddling I got it working (note to self, email them about issues I had getting it set up).

So far it's still in the learning stage. A fair number of false positives and negatives, but the documentation indicates that is normal in the early stages of training. I'll report back as more time passes and see how well it works. If it works quite well I'm thinking of making a set of applescripts for Mail.app so that instead of running it on ones mail server, they can run it on their local machine. I think it should be quite doable.