A mail filter, written in Perl, to identify spam using a wide range of heuristic tests on mail headers and body text. Free software. Can be used as a standalone mail filter, or from procmail. http://spamassassin.apache.org/
Open source automatic mail classification tool using Bayesian statistical methods. Offers details about installation and configuration procedures for the windows client. http://popfile.sourceforge.net/
System of clients and servers that collect and count checksums of email messages in order to detect and filter bulk UCE. http://www.rhyolite.com/anti-spam/dcc/