Nuclear Elephant: DSPAM is a Bayesian spam filter. However, it's one that doesn't just run for a single user, but typically for an entire group of users. I have it running on simon.bofh.ms to scan all the mailboxes there - it integrates well and has a whole range of interesting features. On one hand, there's the web interface for managing the spam filter, and on the other hand, there's the quite pragmatic method for reporting false detections to the filter. Also nice is the quite broad support for databases (MySQL, PostgreSQL, SQLite, and several db* types). Overall, it makes a really well-rounded impression - the only downside is the lack of translation for the interface.
Whether it actually filters well, I of course can't say yet due to lack of volume - the emails first need to accumulate and be trained. User reports are, however - typical for Bayesian spam filters - quite positive.