Linasf is a PHP implementation of a SpamAssassin-like spam filter. It uses Bayesian Filters, User-defined rules and Artificial Neural Networks to obtain excellent results.
Training and integration with current PHP system is very easy. (uses MySQL as DB)
An open-source mail filter program with web-based tuning tool to deliver only wanted mail by using block/buddy lists and naive Bayesian statistical analysis. Gets POP3 mail. Uses php, sh, sendmail, html and javascript. Includes elaborate User Guide.