SpamBayes
The SpamBayes project is working on developing 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. 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. This is documented on the background page.
SpamBayes is not a single application. The core code is a message classifier, however there are several applications available as part of the SpamBayes project which use the classifier in specific contexts. For the most part, the current crop of applications all operate on the client side of things, however, a number of people have experimented with using SpamBayes
