SpamAssassin - Use this program to "Unspam" your mailbox(es).
Mailwasher - Over 8 million people use MailWasher as their spam filter
Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird
Oikik Antispam - Oikik is a cloud-based email filtering service that works with your existing email service.
Spam Reader - Spam Reader is a free anti-spam add-on for Microsoft Outlook.