Bulk Email Filtering

Local POP3 proxies, Windows
http://www.junkspy.com/ - Windows, OS/2 local POP3 proxy

http://hughesnet.net/mailclerk/ - Windows local POP3 proxy

http://www.e-zlegal.com/spamkillerSOFT.html - Windows local POP3 proxy

Local POP3 pre-filters, Windows
http://spameater.co.uk/ - Windows local POP3 filter

http://www.spamkiller.com/

http://www.buypin.com/spamslayer.php - Windows local POP3 filter, adware

Development Efforts
http://www.spamblaster.org/ - POP3 proxy, under development

http://sourceforge.net/projects/pop3filterproxy/ - Java POP3 proxy, under development

Unix
http://www.spambouncer.org/ - procmail-based filtering

http://mailfilter.sourceforge.net/ - POP3 mailbox filtering, mostly *nix, with Windows port

Perl
http://simon-cozens.org/writings/mail-audit.html - Mail::Audit

Spam Assassin - used with Mail::Audit


 * [[Spam Assassin] for exim]

http://adamspiers.org/computing/parp/

Python
TMDA

Distributed Detection
DCC

Vipul's Razor

Other Resources
http://email.about.com/cs/spamfiltering/