Anti-spam framework which incorporates textual analysis, RBLs, Bayesian statistics and on-line spam databases. Works very well indeed -- one big plus of SpamAssassin compared to the purely statistical filters is that it works pretty good straight out of the box, without requiring training.
Here's the page:
For a related script, check out RulesDuJour. It's a very handy script which will check for and download new Spamassassin rules once a day (via cron) and hopefully keep your spam rules fresh, and spam out of your inbox! Be aware that some rules could make your spamassassin memory usage rocket - in particular I found that the BLACKLIST rules are quite large.