Anti-Spam software that is actually worth your while to check out.
See also: SpamConfig, SpamBlacklistsConsideredHarmful, ChallengeResponseSystemsConsideredHarmful
Documentation
Really good discussion on SpamBlacklists on DeclanMcCullagh's PoliTech MailList.
Fairly-Secure Anti-SPAM Gateway Using OpenBSD, Postfix, Amavisd-new, SpamAssassin, Razor and DCC
Anti Spam Tools
- Spam Assassin
web gui for user prefs - http://webuserprefs.pipegrep.net/
Razor (not entirely OpenSource)
- Pyzor
Bayesian Filtering
- Mozilla 1.3+ - I'm currently using this. It seems pretty good, though this build of mozilla seems a bit flaky and prone to crashing.
- Spam Bayes (Advanced Python Bayesian filter)
- POPFile - Not just for spam, POPFile will classify mail into whatever folders you want.