POPFile, a free, open source anti-spam filter,  was one of the first Bayesian filters and is still one of the best. It works as a proxy mail server so that means that it can be used with any POP email client.
As with all Bayesian filters its effectiveness takes a little time to develop as the filter has to be trained to recognize your spam from your normal mail so it may take a week or so before results start becoming accurate and best results may take a month or more.
POPFile is actually an automatic mail classification tool. Once properly set up and trained, it will work in the background of your computer, scanning mail as it arrives and filing it however you wish. You can give it a simple job, like separating out junk e-mail, or a complicated one – like filing mail into a dozen folders. Think of it as a personal assistant for your inbox.
POPFile used to be a recomendation mostly for experienced users, however now it comes in two versions – an easy-to-install Windows version, and a cross-platform version for technically-minded users. Once installed, the two are identical.

Click here to learn more about it and/or download it.

Available for:

• Windows (easy installer)
• Any platform with Perl installed