Spam Assassin

Spam Assassin is an open-source spam killing system produced by the Apache group. Most commonly, it's deployed server-side on a Linux mail server. The code base is publicly available for integration into other products.

It's anti-spam filter uses a Bayesian logic system. This is quite similar to the technology that SpamButcher uses - fuzzy logic. The main difference between the two is that a Bayesian system's data is produced as result of machine based probability analysis of a data set. A fuzzy logic system isn't tied to a specific data source, and isn't necessarily based on probability.

The apache group doesn't currently produce an antispam program for end-users. If you're looking to eliminate unwanted messages - try downloading SpamButcher.

Resource: Spam Assassin