Eigenpoll : Spam fighting software : CRM114

Available at http://crm114.sourceforge.net, this tool uses Bayesian filtering and phrase analysis. It's a bit piggish and therefore unsuitable for server-side filtering with thousands of users (without white/black lists anyway), but it's a perfectly acceptable workload for a system with only tens of users. The filter learns very fast, and achieves (according to the site, and also Paul Graham) well over 99% filter efficiency on a megabyte total training. The threshold can be customized as well.