Also known as an "email filter," a spam filter is software that processes incoming mail in order to attempt to sort out valid communications from unwanted or mass email.
Regular Websites
Open source developed application which uses the Spam Assassin engine to identify unwanted on remote POP3 or IMAP servers. - Spam Cruncher
http://spamcruncher.sourceforge.net/
Offers free, fully-functional challenge-response anti-spam software with no spyware or backdoors for email clients. - Spam Research Center
http://www.spamresearchcenter.com/
Spamato is an openly developed utility which serves as an add-on for Outlook, an extension for Thunderbird and Mozilla Mail, or as a stand-alone application. - Spamato
http://www.spamato.net/
Spamcup is an automatic Spam Cop reporting utility; performing the same actions as if you were to report spam to Spam Cop with a web browser. - Spamcup
http://sourceforge.net/projects/spamcup/
Tantalus is an anti-SPAM filter utility that keeps bulk emailers from guessing usernames by rejecting servers guessing games. - Tantalus
http://www.linuxmailmanager.com/tantalus.html