Mailwasher - Over 8 million people use MailWasher as their spam filter
Spamihilator - The free Anti-Spam-Filter Spamihilator removes more than 98% of spam mails before they even get into your inbox.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird
Spamfence.net - Professional e-mail security free for private use: With Spamfence.
POPFile - POPFile classifies email into categories you define.