Kai's SpamShield
Kai's spamshield ™ for sendmail ™
http://www.spamshield.org/
IPCop Spam Security Project
IPCop Spam Security Project
http://www.ipcop.org/
MailScanner
MailScanner
http://www.mailscanner.info/
MailStripper
Get spam filtering and anti-virus protection for your smtp email server here with our smtp spam filter proxy
http://www.eridani.co.uk/mailstripper/
Sophos PureMessage
Sophos protects against viruses, spyware, adware, phishing, rootkits, and controls network access, web browsing, voip, games, im and spam on a wide range of platforms.
http://www.sophos.com/products/pm/?_x=3
SpamAssassin
SpamAssassin
http://spamassassin.apache.org/index.html