The free
SpamBayes project is working on developing a Bayesian anti-spam filter, initially based on the work of Paul Graham. The major difference between this and other, similar projects is the emphasis on testing newer approaches to scoring messages. While most anti-spam projects are still working with the original graham algorithm, we found that a number of alternate methods yielded a more useful response.
SpamBayes is available as an Outlook add-in version, as well as stand-alone software for Unix/Linux, and as a server application for Mac OS X and Mac OS 9.
My inbox folder, previously spammed to death, looks already much better since installing this little tool.