Bayesian filtering

Bayesian filtering is the process of using Bayesian statistical methods to classify text documents into one of several categories. Bayesian filtering gained currency when it was described in the paper "A Plan for Spam" by Paul Graham.

Links:
Bayes' Theorem
Better Bayesian Filtering - improvements to the algorithm in "A Plan For Spam".
SpamArchive.org - a community resource that provides a database of known spam to be used for testing, developing, and benchmarking anti-spam tools. Donate your spam!
POPFile - automatically sorts your messages and fights spam.
Mozilla Thunderbird - Mozilla's next generation e-mail client. Also uses bayesian filtering.