SpamAssassin is a very flexible Spam filter that employs (amongst other methods) Bayesian classification to identify Spam messages. It is commonly used with procmail (for individual users) or just about any SMTP MTA.
Resources:
- Tweaks for OS X Server - useful for generic purposes as well.
- More Tweaks, which include setting up Vipul's Razor.
- Tools for OS X Server - includes spamtrainer, a nice script that can be tweaked for generic installs.