A statistical (commonly, although a little inaccurately, referred to as 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.
The recommended version for Mac OS X is a POP proxy server, through which email will pass on the way to your desktop email client. As it passes through, SpamBayes tags each message as spam, ham, or unsure. A web-based interface enables you to configure the server through your browser, and feed it spam/ham to improve the accuracy of the filter.