SpamAssassin is an integral part of Maia, which scans messages and assigns them a score based on a combination of heuristics (feature recognizers), DNSBLs, URIBLs, collaborative anti-spam networks, and a Bayesian classifier. It can be found at its official site.

Paul Westbrook shares his experience with upgrading SpamAssassin to 3.0

Additional rule sets for SpamAssassin can be found at the SpamAssassin Rules Emporium (SARE).