Mailwasher - Over 8 million people use MailWasher as their spam filter
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird
POPFile - POPFile classifies email into categories you define.
Spam Reader - Spam Reader is a free anti-spam add-on for Microsoft Outlook.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
SpamSieve - Save time by adding powerful spam filtering to the e-mail client on your Mac.