SpamPal

SpamPal sits between your email program and your mailbox, checking your email as you retrieve it. Any email messages that SpamPal considers to be spam will be "tagged" with a special header; you simply configure your email client to filter anything with this header into a separate folder and your spam won't be mixed up with the rest of your email anymore! More than one mailbox? Don't worry, SpamPal supports multiple POP3 servers, APOP authentication, can filter IMAP4 mailboxes and can even protect an SMTP connection! Extend SpamPal with third-party Plug-ins such as Bayesian filters.
*Eugene's Pick!
N.B. this program is still in beta but is close to first release, any problems need to be reported properly so please read up on it before downloading.

http://www.spampal.org/